feat(pwa): register as an Android share target (#155)

public/manifest.json declares share_target (POST multipart to
/share-target: title/text/url + image/video/audio/pdf/text files). The
service worker answers that POST itself: it stashes the form in a Cache
API bucket and 303s to the in-app /share page, which lists what arrived,
offers a room search, and on pick writes the files into that room's
upload-board atom (encrypting first for E2EE rooms via the composer's
shared filesToUploadItems) and the title/text/url into its draft, then
opens the room — the user still presses Send. The stash is cleared once
placed; reopening /share afterwards says so.

nginx/caddy examples and the prod image config gain a 303 for
/share-target so a POST that reaches the origin before the worker
controls the page lands on /share instead of a 405. iOS has no share
target support and ignores the manifest entry.

Verified headless against the built preview: SW-controlled page → POST
/share-target (two PNGs + title + text) → /share lists both files and the
text → pick the DM → composer shows both files on the upload board and
the text in the draft → /share reports nothing pending.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/pages/Router.tsx

@@ -63,10 +63,14 @@ import { UserRoomProfileRenderer } from '../components/UserRoomProfileRenderer';
 import { HomeCreateRoom } from './client/home/CreateRoom';
 import { Create } from './client/create';
 import { getFallbackSession } from '../state/sessions';
+import { SHARE_PAGE_PATH } from '../../swShare';
+
 import { RouteError } from './RouteError';
 import { CallStatusRenderer } from './CallStatusRenderer';
 import { CallEmbedProvider } from '../components/CallEmbedProvider';
 
+const Share = React.lazy(() => import('./client/share/Share').then((m) => ({ default: m.Share })));
+
 const AuthLayout = React.lazy(() => import('./auth').then((m) => ({ default: m.AuthLayout })));
 const Login = React.lazy(() => import('./auth').then((m) => ({ default: m.Login })));
 const Register = React.lazy(() => import('./auth').then((m) => ({ default: m.Register })));
@@ -380,6 +384,15 @@ export const createRouter = (clientConfig: ClientConfig, screenSize: ScreenSize)
             </React.Suspense>
           }
         />
+        {/* [Gitea #155] PWA share-target landing page. */}
+        <Route
+          path={SHARE_PAGE_PATH}
+          element={
+            <React.Suspense fallback={null}>
+              <Share />
+            </React.Suspense>
+          }
+        />
         <Route
           path={INBOX_PATH}
           element={