remove standalone subtitle extract, unify done semantics, fix nav active matching

Subtitle extraction lives only in the pipeline now; a file is 'done' when it
matches the desired end state — no embedded subs AND audio matches the
language config. The separate Extract page was redundant.

- delete src/routes/review/subtitles/extract.tsx + SubtitleExtractPage
- delete /api/subtitles/extract-all + /:id/extract endpoints
- delete buildExtractOnlyCommand + unused buildExtractionOutputs from ffmpeg.ts
- detail page: drop Extract button + extractCommand textarea, replace with
  'will be extracted via pipeline' note when embedded subs present
- pipeline endpoint: doneCount = is_noop OR status='done' (a file in the
  desired state, however it got there); UI label 'N files in desired state'
- nav: drop the now-defunct 'Extract subs' link, default activeOptions.exact
  to false so detail subpages (e.g. /review/audio/123) highlight their
  parent ('Audio') in the menu — was the cause of the broken-feeling menu

src/routes/__root.tsx

@@ -9,7 +9,7 @@ export const Route = createRootRoute({
 	component: RootLayout,
 });
 
-function NavLink({ to, children }: { to: string; children: React.ReactNode }) {
+function NavLink({ to, children, exact = false }: { to: string; children: React.ReactNode; exact?: boolean }) {
 	return (
 		<Link
 			to={to}
@@ -17,7 +17,7 @@ function NavLink({ to, children }: { to: string; children: React.ReactNode }) {
 				"px-2.5 py-1 rounded text-[0.85rem] no-underline transition-colors text-gray-500 hover:bg-gray-100 hover:text-gray-900",
 			)}
 			activeProps={{ className: "bg-gray-100 text-gray-900 font-medium" }}
-			activeOptions={{ exact: true }}
+			activeOptions={{ exact }}
 		>
 			{children}
 		</Link>
@@ -64,12 +64,13 @@ function RootLayout() {
 				</Link>
 				<VersionBadge />
 				<div className="flex flex-wrap items-center gap-0.5">
-					<NavLink to="/">Dashboard</NavLink>
+					<NavLink to="/" exact>
+						Dashboard
+					</NavLink>
 					<NavLink to="/scan">Scan</NavLink>
 					<NavLink to="/pipeline">Pipeline</NavLink>
 					<NavLink to="/review/audio">Audio</NavLink>
-					<NavLink to="/review/subtitles/extract">Extract subs</NavLink>
-					<NavLink to="/review/subtitles">Subtitle mgr</NavLink>
+					<NavLink to="/review/subtitles">Subtitles</NavLink>
 					<NavLink to="/execute">Jobs</NavLink>
 				</div>
 				<div className="flex-1" />