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/app.rs

@@ -6,7 +6,7 @@ use mumble_web2_common::{ClientConfig, ServerStatus};
 use ordermap::OrderSet;
 use std::collections::{HashMap, HashSet};
 
-use crate::imp;
+use crate::imp::{Platform, PlatformInterface as _};
 
 pub type ChannelId = u32;
 pub type UserId = u32;
@@ -690,12 +690,12 @@ pub fn LoginView(config: Resource<ClientConfig>) -> Element {
     use_resource(move || async move {
         let client = reqwest::Client::new();
         loop {
-            *last_status.write_unchecked() = Some(imp::get_status(&client).await);
-            imp::sleep(std::time::Duration::from_secs_f32(1.0)).await;
+            *last_status.write_unchecked() = Some(Platform::get_status(&client).await);
+            Platform::sleep(std::time::Duration::from_secs_f32(1.0)).await;
         }
     });
 
-    let mut address_input = use_signal(|| imp::load_server_url());
+    let mut address_input = use_signal(|| Platform::load_server_url());
     let address = use_memo(move || {
         if let Some(addr) = address_input() {
             addr.clone()
@@ -706,14 +706,14 @@ pub fn LoginView(config: Resource<ClientConfig>) -> Element {
         }
     });
 
-    let previous_username = imp::load_username();
+    let previous_username = Platform::load_username();
     let mut username = use_signal(|| previous_username.unwrap_or(String::new()));
 
     let do_connect = move |_| {
         //let _ = set_default_username(&username.read());
-        let _ = imp::set_default_username(&username.read());
+        let _ = Platform::set_default_username(&username.read());
         if config.read().as_ref().is_some_and(|cfg| cfg.any_server) {
-            imp::set_default_server(&address.read());
+            Platform::set_default_server(&address.read());
         }
         net.send(Connect {
             address: address.read().clone(),
@@ -860,13 +860,13 @@ pub fn app() -> Element {
 
     use_coroutine(|rx: UnboundedReceiver<Command>| super::network_entrypoint(rx));
     let config = use_resource(|| async move {
-        match imp::load_config().await {
+        match Platform::load_config().await {
             Ok(config) => config,
             Err(_) => ClientConfig::default(),
         }
     });
 
-    imp::request_permissions();
+    Platform::request_permissions();
 
     rsx!(
         document::Link{ rel: "stylesheet", href: "https://fonts.googleapis.com/css2?family=Nunito:ital,wght@0,200..1000;1,200..1000&display=swap" }