Create new generic config abstraction (#25)

This change migrates the config logic to a new generic key+value abstraction. This allows config parameters to be get and set with arbitrary string keys. Config value types can be anything that serde knows how to serialize / deserialize.

Implementations:
Desktop:
Uses a json file in a platform specific directory (pulled from etcetera). This is mostly the same as the existing code. Implemented in `native_config.rs`
Android:
Uses the same mechanism as desktop, with a different path selection that calls out to the android apis (via jni) to get the correct directory.
Web:
Uses browser local storage. Values are stored as strings instead of actual json objects to keep things simple for now. We might want to update this at some point.

Desktop support:
![2026-03-04-223906_grim.png](/attachments/18bfea3e-456c-40f3-9b14-f865c062fcb0)
```
% cat ~/.config/mumble-web2/config.json
{
  "username": "restitux-test",
  "server_url": "voip.ohea.xyz"
}%
```
Web support:
![2026-03-04-223243_grim.png](/attachments/fc55c0c0-1422-4ae8-8e43-9829c6ab7920)
Android support:
![image.png](/attachments/28d9c0f1-ef87-4561-83db-9e4916208267)
```
root@c053bdd1b4da:/# adb shell
tokay:/ $ run-as xyz.ohea.mumble_web_2
tokay:/data/user/0/xyz.ohea.mumble_web_2 $ ls
app_textures  app_webview  cache  code_cache  files  no_backup  shared_prefs
tokay:/data/user/0/xyz.ohea.mumble_web_2 $ ls files
config.json  oat  permission_manager.dex
tokay:/data/user/0/xyz.ohea.mumble_web_2 $ cat files/config.json
{
  "server_url": "voip.ohea.xyz",
  "username": "test"
}tokay:/data/user/0/xyz.ohea.mumble_web_2 $
```

Reviewed-on: #25
Reviewed-by: Sam Sartor <cap@samsartor.com>

gui/src/imp/stub.rs

@@ -10,6 +10,7 @@ pub struct StubPlatform;
 
 impl super::PlatformInterface for StubPlatform {
     type AudioSystem = StubAudioSystem;
+    type ConfigSystem = StubConfigSystem;
 
     fn init_logging() {
         panic!("stubbed platform")
@@ -38,22 +39,6 @@ impl super::PlatformInterface for StubPlatform {
         async { panic!("stubbed platform") }
     }
 
-    fn load_username() -> Option<String> {
-        panic!("stubbed platform")
-    }
-
-    fn load_server_url() -> Option<String> {
-        panic!("stubbed platform")
-    }
-
-    fn set_default_username(_username: &str) -> Option<()> {
-        panic!("stubbed platform")
-    }
-
-    fn set_default_server(_server: &str) -> Option<()> {
-        panic!("stubbed platform")
-    }
-
     fn sleep(_duration: std::time::Duration) -> impl Future<Output = ()> {
         async { panic!("stubbed platform") }
     }
@@ -92,6 +77,28 @@ impl super::AudioPlayerInterface for StubAudioPlayer {
     }
 }
 
+pub struct StubConfigSystem;
+
+impl super::ConfigSystemInterface for StubConfigSystem {
+    fn new() -> Result<Self, Error> {
+        panic!("stubbed platform")
+    }
+
+    fn config_get<T>(&self, key: &str) -> Option<T>
+    where
+        T: serde::de::DeserializeOwned,
+    {
+        panic!("stubbed platform")
+    }
+
+    fn config_set<T>(&self, key: &str, value: &T)
+    where
+        T: serde::Serialize,
+    {
+        panic!("stubbed platform")
+    }
+}
+
 #[allow(unused)]
 pub struct SpawnHandle;