feat(desktop): port v0.11.4-v0.15.2 from Tauri to Electron + Discord-parity audio (v0.16.0)

Chronological port of every Tauri release commit (v0.11.4 -> v0.15.2)
into the Electron rebuild, plus Discord-style audio handling that goes
beyond the Tauri original.

Highlights:
  - All 17 Tauri release commits ported (audio fixes, custom notification
    sound, Discord-style chat UX, profile banner, changelog page,
    Discord-parity call UX, screen-share echo + re-watch UX, audio-loop
    fix).
  - Native napi-rs audio-loopback addon with WASAPI process-loopback:
      * EXCLUDE_TARGET_PROCESS_TREE for full-screen shares -> peers
        never hear themselves echoed back through the capture.
      * INCLUDE_TARGET_PROCESS_TREE for window shares -> only the
        picked window's audio is captured, not the whole OS mixer
        (Discord parity).
      * HWND -> PID resolution via Win32 GetWindowThreadProcessId.
  - Discord-style screen-source picker (thumbnail grid, screens vs
    apps tabs, live-refreshing thumbnails).
  - Hash routing fix for packaged builds (file:// can't resolve
    BrowserRouter paths).
  - Tauri sources removed (apps/desktop/src-tauri).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/desktop/electron/modules/screen-audio.ts

@@ -0,0 +1,38 @@
+// System-audio loopback — renderer-driven. Chromium's
+// `chromeMediaSource: 'desktop'` constraint on getUserMedia accepts a
+// source id and returns a MediaStream that contains the OS mixer output.
+// Main's only job is to resolve which source id the renderer should feed
+// to getUserMedia (typically the primary screen). See the display-media
+// handler in main.ts which grants the request automatically.
+
+import { desktopCapturer, ipcMain } from 'electron';
+
+import { CHANNELS } from '../ipc-types';
+
+export interface ResolveLoopbackSourceResult {
+  sourceId: string;
+}
+
+export function register(): void {
+  ipcMain.handle(
+    CHANNELS.AUDIO_LOOPBACK_RESOLVE_SOURCE,
+    async (): Promise<ResolveLoopbackSourceResult | null> => {
+      // Enumerate only screens; windows don't expose audio loopback on
+      // Windows and there's no meaningful "system audio" tied to a
+      // single window anyway. Primary screen is the first entry — the
+      // id is stable across calls as long as the display config doesn't
+      // change mid-session.
+      const sources = await desktopCapturer.getSources({
+        types: ['screen'],
+        fetchWindowIcons: false,
+      });
+      const first = sources[0];
+      if (!first) return null;
+      return { sourceId: first.id };
+    },
+  );
+
+  // The legacy start/stop channels are left unregistered on purpose —
+  // their constants still exist in ipc-types.ts for backwards compatible
+  // import paths, but there is no corresponding handler.
+}