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/utils/uploadItems.ts

@@ -0,0 +1,22 @@
+import { Room } from 'matrix-js-sdk';
+import { TUploadItem } from '../state/room/roomInputDrafts';
+import { encryptFile } from './matrix';
+import { safeFile } from './mimeTypes';
+import { fulfilledPromiseSettledResult } from './common';
+
+/**
+ * Files → composer upload items for `room`, encrypting first when the room
+ * is E2EE. Shared by the composer's pick/drop/paste paths and the PWA share
+ * target (#155).
+ */
+export async function filesToUploadItems(room: Room, files: File[]): Promise<TUploadItem[]> {
+  const safeFiles = files.map(safeFile);
+  const metadata = { markedAsSpoiler: false };
+  if (room.hasEncryptionStateEvent()) {
+    const encrypted = fulfilledPromiseSettledResult(
+      await Promise.allSettled(safeFiles.map((f) => encryptFile(f))),
+    );
+    return encrypted.map((ef) => ({ ...ef, metadata }));
+  }
+  return safeFiles.map((f) => ({ file: f, originalFile: f, encInfo: undefined, metadata }));
+}