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/mobile.rs

@@ -1,61 +1,85 @@
-use android_permissions::{PermissionManager, RECORD_AUDIO};
-use jni::{objects::JObject, JavaVM};
-use mumble_web2_common::ClientConfig;
+use crate::app::Command;
+use color_eyre::eyre::Error;
+use dioxus::hooks::UnboundedReceiver;
+use mumble_web2_common::{ClientConfig, ServerStatus};
+use std::future::Future;
+use std::time::Duration;
 
-use std::collections::HashMap;
-pub use tokio::runtime::Handle as SpawnHandle;
-pub use tokio::task::spawn;
-pub use tokio::time::sleep;
+/// Mobile platform implementation using Tokio, native audio, and Android permissions.
+pub struct MobilePlatform;
 
-pub use super::connect::*;
-pub use super::native_audio::*;
+impl super::PlatformInterface for MobilePlatform {
+    type AudioSystem = super::native_audio::NativeAudioSystem;
 
-pub fn set_default_username(username: &str) -> Option<()> {
-    None
+    async fn load_config() -> color_eyre::Result<ClientConfig> {
+        Ok(ClientConfig {
+            proxy_url: None,
+            cert_hash: None,
+            any_server: true,
+        })
+    }
+
+    fn load_username() -> Option<String> {
+        None
+    }
+
+    fn load_server_url() -> Option<String> {
+        None
+    }
+
+    fn set_default_username(_username: &str) -> Option<()> {
+        None
+    }
+
+    fn set_default_server(server: &str) -> Option<()> {
+        None
+    }
+
+    async fn network_connect(
+        address: String,
+        username: String,
+        event_rx: &mut UnboundedReceiver<Command>,
+        gui_config: &ClientConfig,
+    ) -> Result<(), Error> {
+        super::connect::network_connect(address, username, event_rx, gui_config).await
+    }
+
+    async fn get_status(client: &reqwest::Client) -> color_eyre::Result<ServerStatus> {
+        super::connect::get_status(client).await
+    }
+
+    fn init_logging() {
+        use tracing::level_filters::LevelFilter;
+        use tracing_subscriber::filter::EnvFilter;
+
+        let env_filter = EnvFilter::builder()
+            .with_default_directive(LevelFilter::INFO.into())
+            .from_env_lossy();
+
+        tracing_subscriber::fmt()
+            .with_target(true)
+            .with_level(true)
+            .with_env_filter(env_filter)
+            .init();
+    }
+
+    fn request_permissions() {
+        request_recording_permission();
+    }
+
+    async fn sleep(duration: Duration) {
+        tokio::time::sleep(duration).await;
+    }
 }
 
-pub fn set_default_server(server: &str) -> Option<()> {
-    None
-}
-
-pub fn load_username() -> Option<String> {
-    None
-}
-
-pub fn load_server_url() -> Option<String> {
-    None
-}
-
-pub async fn load_config() -> color_eyre::Result<ClientConfig> {
-    Ok(ClientConfig {
-        proxy_url: None,
-        cert_hash: None,
-        any_server: true,
-    })
-}
-
-pub fn init_logging() {
-    use tracing::level_filters::LevelFilter;
-    use tracing_subscriber::filter::EnvFilter;
-
-    let env_filter = EnvFilter::builder()
-        .with_default_directive(LevelFilter::INFO.into())
-        .from_env_lossy();
-
-    tracing_subscriber::fmt()
-        .with_target(true)
-        .with_level(true)
-        .with_env_filter(env_filter)
-        .init();
-}
-
-#[cfg(feature = "mobile")]
-pub fn request_permissions() {
-    request_recording_permission();
-}
+#[cfg(not(target_os = "android"))]
+pub fn request_recording_permission() {}
 
 #[cfg(target_os = "android")]
 pub fn request_recording_permission() {
+    use android_permissions::{PermissionManager, RECORD_AUDIO};
+    use jni::{objects::JObject, JavaVM};
+
     let ctx = ndk_context::android_context();
     let vm = unsafe { JavaVM::from_raw(ctx.vm().cast()).unwrap() };
     let activity = unsafe { JObject::from_raw(ctx.context().cast()) };