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]>

components/upload-modal.html

@@ -376,8 +376,9 @@
                      <!-- Additional Technical Sections (Web Seeds, etc) -->
                      <div class="grid gap-3 md:grid-cols-2">
                          <div class="space-y-2">
-                             <label class="block text-xs font-medium text-muted-strong">Web Seed (ws)</label>
-                             <input id="input-ws" type="url" class="input text-sm" placeholder="https://..." />
+                             <label class="block text-xs font-medium text-muted-strong">Web seeds (one per line)</label>
+                             <textarea id="input-ws" rows="2" class="textarea text-sm" placeholder="https://cdn.example.com/video.mp4&#10;https://backup.example.com/video.mp4"></textarea>
+                             <p class="text-xs text-muted">Add a backup origin so the swarm can keep streaming if the primary is overloaded. Each must serve the exact same file.</p>
                          </div>
                           <div class="space-y-2">
                              <label class="block text-xs font-medium text-muted-strong">Torrent File (xs)</label>