feat: configurable auto-update check frequency

Adds a "Settings > Auto-update check" submenu with Disabled / Hourly /
Daily / Weekly. Hourly is the default; existing settings files pick it
up automatically via serde default. Manual "Check for updates" is
unchanged and still fires when auto is disabled.

The 24-hour hardcoded interval is replaced by reading
Settings.update_check_interval_secs in both the startup scheduler and
the post-check rearm path. None means auto is disabled and no timer is
armed.

Adds five new i18n keys across all eight locales.

README.md

@@ -83,8 +83,9 @@ corner of your primary monitor on first launch. Drag it where you want it,
 release to snap to the nearest edge if you let go close to one.
 
 - **Left-click** the bubble to open the expanded panel (5h + 7d + countdowns)
-- **Right-click** for refresh, models, update frequency, language, "Start
-  with Windows", updates, exit
+- **Right-click** for refresh, models, refresh frequency, language, "Start
+  with Windows", auto-update check (Disabled / Hourly / Daily / Weekly),
+  manual "Check for updates", exit
 - **Drag** anywhere — it floats on top of all other windows
 - **Ctrl + MouseWheel** on the bubble to resize it
 - **Tray icon** (if enabled): left-click toggles the bubble visibility,