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/commands.rs

@@ -2430,3 +2430,33 @@ pub async fn set_onboarding_completed(app: AppHandle, completed: bool) -> Result
     )
     .map_err(|e| e.to_string())
 }
+
+const LAST_SEEN_VERSION_FILE: &str = "settings/last_seen_version.txt";
+
+/// The app version recorded on the previous launch. `None` when the file is
+/// absent (fresh install, or first launch since this was introduced) — the
+/// frontend uses that to decide whether to surface the "What's New" prompt.
+#[tauri::command]
+pub async fn get_last_seen_version(app: AppHandle) -> Result<Option<String>, String> {
+    let app_dir = app.path().app_data_dir().map_err(|e| e.to_string())?;
+    let path = app_dir.join(LAST_SEEN_VERSION_FILE);
+    if !path.exists() {
+        return Ok(None);
+    }
+    let content = std::fs::read_to_string(&path).map_err(|e| e.to_string())?;
+    let trimmed = content.trim();
+    Ok(if trimmed.is_empty() {
+        None
+    } else {
+        Some(trimmed.to_string())
+    })
+}
+
+#[tauri::command]
+pub async fn set_last_seen_version(app: AppHandle, version: String) -> Result<(), String> {
+    let app_dir = app.path().app_data_dir().map_err(|e| e.to_string())?;
+    let settings_dir = app_dir.join("settings");
+    std::fs::create_dir_all(&settings_dir).map_err(|e| e.to_string())?;
+    std::fs::write(settings_dir.join("last_seen_version.txt"), version.trim())
+        .map_err(|e| e.to_string())
+}

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,
         ])