feat(ui): About dialog, Release Notes, and new-version banner

About (in the account menu) shows frontend/backend/database versions + build.
Release Notes renders per-version highlights with a commit-SHA reference; a
dismissible banner appears once after the running build's version changes and
links into the notes. Adds a reusable Modal shell and the release-notes data
(detailed v0.1.0).

frontend/src/components/Header.tsx

@@ -1,5 +1,5 @@
 import { useRef, useState } from "react";
-import { BarChart3, Home, LogOut, Search, Settings, Shield, Tv } from "lucide-react";
+import { BarChart3, Home, Info, LogOut, Search, Settings, Shield, Tv } from "lucide-react";
 import type { FeedFilters, Me } from "../lib/api";
 import type { Page } from "../lib/urlState";
 import SyncStatus from "./SyncStatus";
@@ -13,6 +13,7 @@ export default function Header({
   page,
   setPage,
   onOpenSettings,
+  onOpenAbout,
   onGoToFullHistory,
 }: {
   me: Me;
@@ -21,6 +22,7 @@ export default function Header({
   page: Page;
   setPage: (p: Page) => void;
   onOpenSettings: () => void;
+  onOpenAbout: () => void;
   onGoToFullHistory: () => void;
 }) {
   async function logout() {
@@ -65,6 +67,7 @@ export default function Header({
             page={page}
             setPage={setPage}
             onOpenSettings={onOpenSettings}
+            onOpenAbout={onOpenAbout}
           />
         </div>
       </div>
@@ -88,12 +91,14 @@ function AccountMenu({
   page,
   setPage,
   onOpenSettings,
+  onOpenAbout,
 }: {
   me: Me;
   logout: () => void;
   page: Page;
   setPage: (p: Page) => void;
   onOpenSettings: () => void;
+  onOpenAbout: () => void;
 }) {
   const [open, setOpen] = useState(false);
   const closeTimer = useRef<ReturnType<typeof setTimeout> | null>(null);
@@ -161,6 +166,10 @@ function AccountMenu({
               <Settings className="w-4 h-4" />
               Settings
             </button>
+            <button onClick={() => { onOpenAbout(); setOpen(false); }} className={itemClass}>
+              <Info className="w-4 h-4" />
+              About
+            </button>
             <button onClick={logout} className={itemClass}>
               <LogOut className="w-4 h-4" />
               Sign out