refactor: remove Tauri auto-updater, replace with simple GitHub-check modal

Remove tauri-plugin-updater, relaunch_after_update command, generate-update-
manifest.js, the generate-manifest CI job, and all signing/upload steps for
.sig files and latest.json. The updater plugin, its pubkey and endpoint config
in tauri.conf.json, and the updater:default capability are all gone.

Replace AppUpdater.tsx with a lightweight component that:
- Fetches https://api.github.com/repos/Psychotoxical/psysonic/releases/latest
  after a 4 s delay (no install, no download, no progress bar)
- Shows a dismissible toast with two buttons:
  GitHub → releases/latest
  Website → https://psysonic.psychotoxic.eu/#downloads

i18n: remove updaterInstall/Downloading/Installing/Download/ExperimentalHint
keys from all 7 locales; add updaterWebsite; update updaterVersion wording.

CI: remove generate-manifest job and all .sig signing/upload steps from
build-macos-windows. Also remove TAURI_SIGNING_PRIVATE_KEY env vars (no
longer needed). Release now just builds and uploads the installable binaries.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src-tauri/src/lib.rs

@@ -36,49 +36,6 @@ fn exit_app(app_handle: tauri::AppHandle) {
     app_handle.exit(0);
 }
 
-/// Restart after an in-app update.
-///
-/// `relaunch()` from tauri-plugin-process spawns the new process while the old one
-/// is still alive, so the single-instance plugin in the new process sees the old
-/// socket and kills itself immediately (endless loop).
-///
-/// This command instead:
-///   1. Spawns a shell snippet that waits ~1.5 s and then opens the app again —
-///      by then the old process and its single-instance socket are fully gone.
-///   2. Calls `app.exit(0)` directly, which bypasses `on_window_event`
-///      prevent_close() and releases the single-instance lock immediately.
-#[tauri::command]
-fn relaunch_after_update(app: tauri::AppHandle) {
-    #[cfg(target_os = "windows")]
-    {
-        let exe = std::env::current_exe().unwrap_or_default();
-        let exe_str = exe.to_string_lossy().to_string().replace('\'', "''");
-        let script = format!(
-            "Start-Sleep -Milliseconds 1500; Start-Process '{exe_str}'"
-        );
-        let _ = std::process::Command::new("powershell")
-            .args(["-WindowStyle", "Hidden", "-NonInteractive", "-Command", &script])
-            .spawn();
-    }
-
-    #[cfg(target_os = "macos")]
-    {
-        let exe = std::env::current_exe().unwrap_or_default();
-        // exe lives at Psysonic.app/Contents/MacOS/psysonic — walk up to the bundle.
-        let bundle = exe
-            .parent()                  // MacOS/
-            .and_then(|p| p.parent())  // Contents/
-            .and_then(|p| p.parent()); // Psysonic.app
-        if let Some(bundle_path) = bundle {
-            let escaped = bundle_path.to_string_lossy().replace('"', "\\\"");
-            let _ = std::process::Command::new("sh")
-                .args(["-c", &format!("sleep 1.5 && open \"{escaped}\"")])
-                .spawn();
-        }
-    }
-
-    app.exit(0);
-}
 
 /// Authenticate with Navidrome's own REST API and return a Bearer token.
 async fn navidrome_token(server_url: &str, username: &str, password: &str) -> Result<String, String> {
@@ -854,7 +811,6 @@ pub fn run() {
         .manage(ShortcutMap::default())
         .manage(discord::DiscordState::new())
         .manage(TrayState::default())
-        .plugin(tauri_plugin_updater::Builder::new().build())
         .plugin(tauri_plugin_process::init())
         .plugin(tauri_plugin_window_state::Builder::default().build())
         .plugin(tauri_plugin_shell::init())
@@ -1031,7 +987,6 @@ pub fn run() {
             get_hot_cache_size,
             delete_hot_cache_track,
             purge_hot_cache,
-            relaunch_after_update,
             toggle_tray_icon,
         ])
         .run(tauri::generate_context!())