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

desktop/src-tauri/src/local/mod.rs

@@ -24,3 +24,17 @@ pub fn open(path: &Path) -> rusqlite::Result<Db> {
     schema::migrate(&conn)?;
     Ok(Db(Mutex::new(conn)))
 }
+
+/// A one-line count summary of the store, for the startup log.
+pub fn summary(db: &Db) -> String {
+    let conn = match db.0.lock() {
+        Ok(c) => c,
+        Err(_) => return "counts unavailable (lock poisoned)".to_string(),
+    };
+    let count = |sql: &str| conn.query_row(sql, [], |r| r.get::<_, i64>(0)).unwrap_or(-1);
+    format!(
+        "{} notes, {} labels",
+        count("SELECT COUNT(*) FROM notes"),
+        count("SELECT COUNT(*) FROM labels"),
+    )
+}