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/swShare.test.ts

@@ -0,0 +1,27 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { shareDraftText } from './swShare';
+
+const base = { files: [], receivedAt: 0 };
+
+test('shareDraftText joins title, text and url', () => {
+  assert.equal(
+    shareDraftText({ ...base, title: 'A page', text: 'look at this', url: 'https://x.test/p' }),
+    'A page\nlook at this\nhttps://x.test/p',
+  );
+});
+
+test('shareDraftText drops a title/url the text already contains', () => {
+  assert.equal(
+    shareDraftText({
+      ...base,
+      title: 'A page',
+      text: 'A page https://x.test/p',
+      url: 'https://x.test/p',
+    }),
+    'A page https://x.test/p',
+  );
+  assert.equal(shareDraftText({ ...base, text: 'from the share sheet' }), 'from the share sheet');
+  assert.equal(shareDraftText({ ...base, url: 'https://x.test' }), 'https://x.test');
+  assert.equal(shareDraftText(base), '');
+});