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.

src/i18n/mod.rs

@@ -43,6 +43,11 @@ pub struct LocaleStrings {
     pub applying_update: String,
     pub update_available: String,
     pub update_via_winget: String,
+    pub auto_update_check: String,
+    pub auto_check_disabled: String,
+    pub auto_check_hourly: String,
+    pub auto_check_daily: String,
+    pub auto_check_weekly: String,
     pub exit: String,
     pub show_widget: String,
     pub session_window: String,