feat: What's New screen + updater progress, plus light-theme fixes

Post-update UX for the next release (0.1.2):

- What's New: after a version change, greet the user with a toast that opens
  an in-app release-notes screen (collapsible Added/Changed/Fixed sections)
  sourced from the bundled CHANGELOG.md, reopenable from Settings -> Updates.
  Backed by a last_seen_version settings file (get/set_last_seen_version
  commands) that distinguishes upgrades from fresh installs.
- Updater: the download/install progress toast now reappears when an update is
  started from the title-bar indicator or Settings after the prompt was
  dismissed; Settings -> Updates gains a real progress bar with a percentage.
- Light theme: fix the recurring subtle-light breakage. Neutral surfaces now
  rely on the CSS-variable remap instead of light-theme:bg-white (which forced
  surfaces dark because --color-white is remapped dark); the green action
  buttons drop the broken light-theme:hover:bg-emerald-200 (remapped dark,
  unreadable on hover) for an override-free emerald-500 tint that auto-themes,
  across the updater, What's New, and onboarding.
- Debug panel: add What's New triggers (toast / modal / reset).

src-tauri/src/lib.rs

@@ -208,6 +208,8 @@ pub fn run() {
             commands::retry_ffmpeg_download,
             commands::get_onboarding_completed,
             commands::set_onboarding_completed,
+            commands::get_last_seen_version,
+            commands::set_last_seen_version,
             commands::get_notifications_paused,
             commands::set_notifications_paused,
         ])