feat: make update check opt-in via Help menu toggle

[JBolanle]

Summary

• Removes the automatic GitHub API request fired on every app launch
• Adds a Check for Updates Automatically checkable menu item to Help (defaults to off); its state is initialised from saved settings on startup and persists via the existing settings store
• The existing Check for Updates… manual trigger is always available regardless of preference

Changes

• src-tauri/src/settings.rs — new auto_update_check: bool field with #[serde(default)] (backward compatible with existing settings files)
• src-tauri/src/menu.rs — CheckMenuItem + separator added to Help menu; build_menu returns it as a third value
• src-tauri/src/lib.rs — destructures new return value, calls set_checked() from saved settings in setup, wires menu:check-for-update-auto event
• src/lib/stores/settingsStore.ts — auto_update_check: boolean added to AppSettings interface and defaults
• src/routes/+page.svelte — onMount now awaits settings load, conditionally checks for updates, handles menu:check-for-update-auto toggle

Test Plan

• All 90 frontend tests pass (npm test)
• All 42 Rust tests pass (cargo test from src-tauri/)
• Type check passes (npm run check)
• On fresh install: Help → Check for Updates Automatically is unchecked; no network request on launch
• Enable the toggle: next launch auto-checks silently; update toast appears if a new version exists
• Disable the toggle: no network request on next launch
• Help → Check for Updates… always triggers a manual check with "up to date" feedback

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