Refactor the imp/gui bondary to use real traits (#18)

# Summary

Introduces a trait-based platform abstraction layer that makes the boundary
between platform-specific and shared code explicit and compile-time verified.

The TLDR version of this new trait stuff works:

1. Define a `PlatformInterface` trait.
2. Each platform defines a zero-sized struct implementing the trait (ex `WebPlatform`).
3. Create an ifdef'd type alias on those structs:

```rust
#[cfg(feature = "web")]
pub type Platform = web::WebPlatform;

#[cfg(all(feature = "desktop"))]
pub type Platform = desktop::DesktopPlatform;

#[cfg(all(feature = "mobile", not(feature = "web")))]
pub type Platform = mobile::MobilePlatform;
```

5. Add a compile time assertion that `Platform` implements `PlatformInterface`.

#  Motivation

Previously, platform code used a mix of pub use re-exports and #[cfg] blocks
that made it difficult to understand what each platform must implement. The
new trait-based approach provides:

- Clear documentation of the platform contract
- Compile-time verification that all platforms implement required
  functionality
- Ability to cargo check without feature flags (via stub platform)

# Changes

New traits in imp/mod.rs:
  - PlatformInterface - logging, permissions, network, config, storage. Overall this the trait that platforms must satify to compile.
  - AudioSystemInterface - audio system initialization and recording
  - AudioPlayerInterface - opus audio playback

 Type aliases:
  - Platform, AudioSystem, AudioPlayer resolve to the correct types based on
  feature flags

Call site updates:
  - Changed from imp::function() to Platform::function() syntax
  - Removed ImpRead/ImpWrite helper traits in favor of direct bounds

# Testing

Manual testing reveals that Web and Desktop still work, I (Liam) have not tested the mobile version beyond compilation.

Co-authored-by: Liam Warfield <liam.warfield@gmail.com>
Reviewed-on: #18
Co-authored-by: Sam Sartor <me@samsartor.com>
Co-committed-by: Sam Sartor <me@samsartor.com>

gui/src/imp/mod.rs

@@ -1,29 +1,166 @@
-#[cfg(feature = "web")]
-mod web;
+//! Platform abstraction layer
+//!
+//! This module defines traits that each platform (web, desktop, mobile) must implement.
+//! The traits make the platform boundary explicit and provide compile-time verification.
+#![allow(async_fn_in_trait)]
+
+use crate::{app::Command, effects::AudioProcessor};
+use color_eyre::eyre::Error;
+use dioxus::hooks::UnboundedReceiver;
+use mumble_web2_common::{ClientConfig, ServerStatus};
+use std::future::Future;
+use std::time::Duration;
+
+// ============================================================================
+// Trait Definitions
+// ============================================================================
+
+/// Platform-specific audio subsystem for capturing microphone input and creating playback streams.
+///
+/// The audio system handles Opus encoding internally - callers receive encoded frames
+/// ready for network transmission.
+pub trait AudioSystemInterface: Sized {
+    /// The player type returned by [`create_player`](Self::create_player).
+    type AudioPlayer: AudioPlayerInterface;
+
+    /// Initialize the audio system.
+    async fn new() -> Result<Self, Error>;
+
+    /// Set the processor for the microphone input, mainly noise cancellation settings.
+    fn set_processor(&self, processor: AudioProcessor);
+
+    /// Begin listening to microphone input, calling the `each` function with
+    /// encoded opus frames.
+    fn start_recording(
+        &mut self,
+        each: impl FnMut(Vec<u8>, bool) + Send + 'static,
+    ) -> Result<(), Error>;
+
+    /// Begin playback of an audio stream, returning an object that can be passed  opus frames.
+    fn create_player(&mut self) -> Result<Self::AudioPlayer, Error>;
+}
+
+/// A handle to an active audio playback stream for a single remote user.
+///
+/// Each connected user gets their own `AudioPlayer` instance, which decodes
+/// incoming Opus frames and outputs PCM audio to the platform's audio device.
+/// The player manages its own decoder state and output buffer.
+pub trait AudioPlayerInterface {
+    /// Decode and play an Opus-encoded audio frame.
+    fn play_opus(&mut self, payload: &[u8]);
+}
+
+/// This is the main trait that each platform must implement. It combines all
+/// platform-specific functionality into a single interface, providing compile-time
+/// verification that all platforms implement the required functionality.
+pub trait PlatformInterface {
+    type AudioSystem: AudioSystemInterface;
+
+    /// Initialize logging for the platform.
+    fn init_logging();
+
+    /// Request runtime permissions (Android audio recording, etc.).
+    fn request_permissions();
+
+    /// Establish a connection to the Mumble server and run the network loop.
+    fn network_connect(
+        address: String,
+        username: String,
+        event_rx: &mut UnboundedReceiver<Command>,
+        gui_config: &ClientConfig,
+    ) -> impl Future<Output = Result<(), Error>>;
+
+    /// Get server status (user count, version, etc.).
+    fn get_status(
+        client: &reqwest::Client,
+    ) -> impl Future<Output = color_eyre::Result<ServerStatus>>;
+
+    /// Load the proxy overrides (proxy URL, cert hash, etc.).
+    fn load_config() -> impl Future<Output = color_eyre::Result<ClientConfig>>;
+
+    /// Load saved username.
+    fn load_username() -> Option<String>;
+
+    /// Load saved server URL.
+    fn load_server_url() -> Option<String>;
+
+    /// Save the default username.
+    fn set_default_username(username: &str) -> Option<()>;
+
+    /// Save the default server URL.
+    fn set_default_server(server: &str) -> Option<()>;
+
+    /// Async sleep for the given duration.
+    fn sleep(duration: Duration) -> impl Future<Output = ()>;
+}
+
+// ============================================================================
+// Platform Modules
+// ============================================================================
 
 #[cfg(any(feature = "desktop", feature = "mobile"))]
 mod connect;
-#[cfg(any(feature = "desktop", feature = "mobile"))]
-mod native_audio;
-
 #[cfg(feature = "desktop")]
 mod desktop;
 #[cfg(feature = "mobile")]
 mod mobile;
+#[cfg(any(feature = "desktop", feature = "mobile"))]
+mod native_audio;
+mod stub;
+#[cfg(feature = "web")]
+mod web;
 
-#[cfg(feature = "desktop")]
-pub use desktop::*;
-#[cfg(feature = "mobile")]
-pub use mobile::*;
+// ============================================================================
+// Platform Type Alias
+// ============================================================================
 
-#[cfg(feature = "mobile")]
-pub use mobile::request_permissions;
+#[cfg(feature = "web")]
+pub type Platform = web::WebPlatform;
 
-#[cfg(any(feature = "desktop", feature = "web"))]
-pub fn request_permissions() {}
+#[cfg(all(feature = "desktop", not(feature = "web")))]
+pub type Platform = desktop::DesktopPlatform;
 
-#[cfg(all(feature = "web", not(any(feature = "desktop", feature = "mobile"))))]
-pub use web::*;
+#[cfg(all(feature = "mobile", not(feature = "web"), not(feature = "desktop")))]
+pub type Platform = mobile::MobilePlatform;
 
-#[cfg(any(feature = "desktop"))]
-pub use desktop::*;
+#[cfg(all(
+    not(feature = "mobile"),
+    not(feature = "web"),
+    not(feature = "desktop")
+))]
+pub type Platform = stub::StubPlatform;
+
+pub type AudioSystem = <Platform as PlatformInterface>::AudioSystem;
+pub type AudioPlayer = <AudioSystem as AudioSystemInterface>::AudioPlayer;
+
+// ========================
+// Platform Async Runtime
+// ========================
+
+// Note: these can not be part of the Platform because they differ in Send requiremets
+#[cfg(all(any(feature = "desktop", feature = "mobile"), not(feature = "web")))]
+pub use connect::{spawn, SpawnHandle};
+#[cfg(all(
+    not(feature = "desktop"),
+    not(feature = "mobile"),
+    not(feature = "web")
+))]
+pub use stub::{spawn, SpawnHandle};
+#[cfg(feature = "web")]
+pub use web::{spawn, SpawnHandle};
+
+// =======================
+// Compile-time Assertions
+// =======================
+const _: () = {
+    fn assert_platform<T: PlatformInterface>() {}
+
+    // Check each implementation, and prevent warnings that the implementations are unused.
+    #[cfg(feature = "web")]
+    let _ = assert_platform::<web::WebPlatform>;
+    #[cfg(feature = "desktop")]
+    let _ = assert_platform::<desktop::DesktopPlatform>;
+    #[cfg(feature = "mobile")]
+    let _ = assert_platform::<mobile::MobilePlatform>;
+    let _ = assert_platform::<stub::StubPlatform>;
+};