Add Platform trait.

I did some more thinking about the whole trait/boundary stuff and reallized that we don't need the GUI to handle the generic-ness of a trait object since only 1 platform will ever be used in a binary. What do you think of the following:

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

```
     // gui/src/platform/mod.rs
     #[cfg(feature = "web")]
     pub type CurrentPlatform = web::WebPlatform;
     #[cfg(feature = "desktop")]
     pub type CurrentPlatform = desktop::DesktopPlatform;
     #[cfg(feature = "mobile")]
     pub type CurrentPlatform = mobile::MobilePlatform;
```

4. Add a compile time assertion that `CurrentPlatform` implements `Platform`.

Pros:

- We don't end up working around async trait objects
- We define what functions are needed for a platform
- We save a little on binary size by avoiding a fully generic solution.

Cons:

- The trait does not really do much other than being a collection of functions.
- In some ways it seems like what we're currently doing but with extra steps.

gui/src/imp/mod.rs

@@ -1,3 +1,126 @@
+//! 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.
+
+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;
+
+// ============================================================================
+// Trait Definitions
+// ============================================================================
+
+/// Trait for spawn handles that can be stored and used to spawn tasks later.
+#[cfg(feature = "web")]
+pub trait SpawnHandleTrait: Clone + 'static {
+    /// Spawn an async task using this handle.
+    fn spawn<F>(&self, future: F)
+    where
+        F: Future<Output = ()> + 'static;
+
+    /// Get a spawn handle for the current context.
+    fn current() -> Self;
+}
+
+/// Trait for spawn handles that can be stored and used to spawn tasks later.
+#[cfg(any(feature = "desktop", feature = "mobile"))]
+pub trait SpawnHandleTrait: Clone + 'static {
+    /// Spawn an async task using this handle.
+    fn spawn<F>(&self, future: F)
+    where
+        F: Future<Output = ()> + Send + 'static;
+
+    /// Get a spawn handle for the current context.
+    fn current() -> Self;
+}
+
+/// Runtime primitives: task spawning and async sleep.
+#[cfg(feature = "web")]
+pub trait PlatformRuntime {
+    /// The spawn handle type for this platform.
+    type SpawnHandle: SpawnHandleTrait;
+
+    /// Spawn an async task.
+    fn spawn<F>(future: F)
+    where
+        F: Future<Output = ()> + 'static;
+
+    /// Async sleep for the given duration.
+    fn sleep(duration: Duration) -> impl Future<Output = ()>;
+}
+
+/// Runtime primitives: task spawning and async sleep.
+#[cfg(any(feature = "desktop", feature = "mobile"))]
+pub trait PlatformRuntime {
+    /// The spawn handle type for this platform.
+    type SpawnHandle: SpawnHandleTrait;
+
+    /// Spawn an async task.
+    fn spawn<F>(future: F)
+    where
+        F: Future<Output = ()> + Send + 'static;
+
+    /// Async sleep for the given duration.
+    fn sleep(duration: Duration) -> impl Future<Output = ()>;
+}
+
+/// Configuration persistence: loading and saving user preferences.
+pub trait PlatformConfig {
+    /// Load the client configuration (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<()>;
+}
+
+/// Network operations: connecting to servers.
+pub trait PlatformNetwork {
+    /// 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>>;
+}
+
+/// Platform initialization.
+pub trait PlatformInit {
+    /// Initialize logging for the platform.
+    fn init_logging();
+
+    /// Request runtime permissions (Android audio recording, etc.).
+    fn request_permissions();
+}
+
+/// Combined platform trait.
+///
+/// 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 Platform: PlatformRuntime + PlatformConfig + PlatformNetwork + PlatformInit {}
+
+// ============================================================================
+// Platform Modules
+// ============================================================================
+
 #[cfg(feature = "web")]
 mod web;
 
@@ -11,6 +134,30 @@ mod desktop;
 #[cfg(feature = "mobile")]
 mod mobile;
 
+// ============================================================================
+// Platform Type Alias
+// ============================================================================
+
+/// The current platform type, selected at compile time based on features.
+#[cfg(feature = "web")]
+pub type CurrentPlatform = web::WebPlatform;
+
+#[cfg(feature = "desktop")]
+pub type CurrentPlatform = desktop::DesktopPlatform;
+
+#[cfg(feature = "mobile")]
+pub type CurrentPlatform = mobile::MobilePlatform;
+
+/// Compile-time assertion that CurrentPlatform implements Platform.
+const _: () = {
+    fn assert_platform<T: Platform>() {}
+    let _ = assert_platform::<CurrentPlatform>;
+};
+
+// ============================================================================
+// Platform Re-exports
+// ============================================================================
+
 #[cfg(feature = "desktop")]
 pub use desktop::*;
 #[cfg(feature = "mobile")]
@@ -24,6 +171,3 @@ pub fn request_permissions() {}
 
 #[cfg(all(feature = "web", not(any(feature = "desktop", feature = "mobile"))))]
 pub use web::*;
-
-#[cfg(any(feature = "desktop"))]
-pub use desktop::*;