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/components/DemoPanel.tsx

@@ -42,6 +42,7 @@ const DEMO_UPDATE_VERSION = "0.2.0";
 
 export function DemoPanel() {
   const folders = useGalleryStore((state) => state.folders);
+  const appVersion = useGalleryStore((state) => state.appVersion);
   const [open, setOpen] = useState(false);
   const downloadTimer = useRef<number | null>(null);
 
@@ -160,6 +161,18 @@ export function DemoPanel() {
     });
   };
 
+  // --- What's New flow ------------------------------------------------------
+  // Drives the post-update greeting without needing a real version change. The
+  // toast + modal pull their content from the bundled changelog for the running
+  // app version, so the current release's notes are what render.
+
+  const showWhatsNewToast = () =>
+    useGalleryStore.setState({ whatsNewToast: appVersion ?? DEMO_UPDATE_VERSION, whatsNewOpen: false });
+
+  const openWhatsNewModal = () => useGalleryStore.setState({ whatsNewOpen: true, whatsNewToast: null });
+
+  const resetWhatsNew = () => useGalleryStore.setState({ whatsNewOpen: false, whatsNewToast: null });
+
   if (!open) return null;
 
   const injectBtn =
@@ -215,6 +228,24 @@ export function DemoPanel() {
           Reset updater state
         </button>
       </div>
+
+      <div className="my-3 h-px bg-amber-400/20" />
+
+      <p className="mb-1.5 text-[11px] font-semibold uppercase tracking-wide text-amber-200/60">What's new</p>
+      <p className="mb-2 text-[11px] leading-snug text-amber-200/70">
+        Post-update greeting for v{appVersion ?? "—"}, sourced from the bundled changelog.
+      </p>
+      <div className="flex flex-col gap-1.5">
+        <button className={injectBtn} onClick={showWhatsNewToast}>
+          Show "What's new" toast
+        </button>
+        <button className={injectBtn} onClick={openWhatsNewModal}>
+          Open "What's new" modal
+        </button>
+        <button className={neutralBtn} onClick={resetWhatsNew}>
+          Reset What's New state
+        </button>
+      </div>
     </div>
   );
 }