desktop: robust startup + operation logging (portability troubleshooting)

There was essentially no logging — useless for proving the app renders across
different environments. Add real observability:

- tauri-plugin-log -> stdout (so `2>&1 | tee` captures a run) AND a persistent
  file in the app log dir (grabbable after the fact on any machine). Level Info.
- Startup diagnostics: app version, OS/arch, the Linux display/session stack
  (XDG_SESSION_TYPE, desktop, Wayland/X11, GDK_BACKEND), the WebKit render-
  hardening vars actually in effect, resolved log + data dirs, DB open/migrate
  result, and note/label counts.
- log_event command + a frontend logEvent() helper: boot line (data source +
  WebKit user-agent) from main.ts, first-route config/session/destination from
  the router guard, and — via the bridge invoke() wrapper — every failed Tauri
  command named with its error, so a broken basic function is self-identifying.

Task 2040.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

frontend/src/desktop/bridge.ts

@@ -27,7 +27,18 @@ export function isDesktop(): boolean {
 export function invoke<T>(cmd: string, args?: Record<string, unknown>): Promise<T> {
   const tauri = window.__TAURI__;
   if (!tauri) return Promise.reject(new Error("Not running in the desktop app."));
-  return tauri.core.invoke<T>(cmd, args);
+  return tauri.core.invoke<T>(cmd, args).catch((err: unknown) => {
+    // Every failed command names itself in the log — so a broken "basic function"
+    // in some environment is diagnosable. Skip log_event to avoid recursion.
+    if (cmd !== "log_event") logEvent("error", `invoke '${cmd}' failed: ${String(err)}`);
+    throw err;
+  });
+}
+
+/** Route a frontend message into the desktop log (stdout + file). No-op on web. */
+export function logEvent(level: "info" | "warn" | "error" | "debug", message: string): void {
+  if (!isDesktop()) return;
+  void invoke("log_event", { level, message }).catch(() => {});
 }
 
 // Desktop (Linux AppImage) self-integration: add/remove an applications-menu entry.