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/Cargo.toml

@@ -24,6 +24,10 @@ rusqlite = { version = "0.32", features = ["bundled"] }
 uuid = { version = "1", features = ["v4"] }
 # RFC3339 timestamps for created_at/updated_at/remind_at (Date.parse-able on the JS side).
 chrono = { version = "0.4", default-features = false, features = ["clock"] }
+# Startup + operation logging to stdout AND a persistent file, so portability
+# issues are diagnosable from any environment. `log` is the facade the code uses.
+tauri-plugin-log = "2"
+log = "0.4"
 
 # Tauri's default release profile: smaller, faster shipped binaries.
 [profile.release]