feat(webseed): upload form accepts multiple web seeds (slice 2/3)

Let uploaders declare a backup origin at creation time so the swarm can keep
streaming when the primary CDN is overloaded.

- upload-modal.html: the single "Web Seed (ws)" input becomes a "Web seeds
  (one per line)" textarea with help text explaining the backup-origin use
  case and the byte-identical-file requirement.
- UploadModal: in upload mode the R2/CDN url stays the primary webseed and any
  textarea entries follow it (passed as [primaryUrl, textarea]); external mode
  already flowed the field through. All parsing stays in the payload layer.
- videoNotePayload.normalizeWebSeedList now splits every entry (array OR
  string) on newlines/commas, so the upload-modal's [url, "a\nb"] shape and a
  plain multi-line string both expand + dedupe correctly.

Mutation-verified (array entries no longer split fails the upload-shape test);
build + full unit suite green. Lints relevant to the change (inline-styles,
tokens, innerHTML) pass.

Note: npm run lint also reports pre-existing file-size violations in unrelated
files (SignerManager, watchHistory, playbackService, webtorrent, etc.) that
grew in earlier commits — none are touched here. Flagging separately.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

tests/multi-webseed.test.mjs

@@ -81,6 +81,25 @@ test("a single webseed string still works (back-compat)", () => {
   assert.equal(payload.legacyFormData.ws, SEED_A);
 });
 
+test("upload-modal shape ([primaryUrl, textarea]) yields primary-first, all split", () => {
+  // This mirrors exactly what UploadModal hands over in upload mode: the CDN
+  // url as the primary entry, followed by the raw multi-line textarea value.
+  const cdn = "https://cdn.example.com/video.mp4";
+  const { payload } = normalizeVideoNotePayload({
+    title: "Upload shape",
+    url: cdn,
+    magnet: MAGNET,
+    ws: [cdn, `${SEED_A}\n${SEED_B}`],
+  });
+
+  const seeds = extractAllWebSeeds(payload.legacyFormData.magnet);
+  assert.ok(seeds.includes(cdn), "CDN url carried");
+  assert.ok(seeds.includes(SEED_A), "first backup webseed carried");
+  assert.ok(seeds.includes(SEED_B), "second backup webseed carried");
+  // CDN stays the primary (first) seed.
+  assert.equal(payload.legacyFormData.ws, cdn);
+});
+
 test("duplicate webseeds are not written twice", () => {
   const { payload } = normalizeVideoNotePayload({
     title: "Dupe seed",