[edit] Derive a webseed torrent when editing a video's URL to an external link

Mirror the upload-modal external-URL webseed for the Edit modal: when the user
changes a video's URL to an external link with no magnet, auto-derive the webseed
torrent (fetch + hash the remote file → magnet ws=url) so an edited external URL
keeps the P2P benefit. Best-effort + size-capped; on CORS / too-large / any error
it stays URL-only. Runs before the existing magnet normalization so the derived
magnet's ws/xs are extracted the same way a pasted magnet's are.

submit() is now async (it was fire-and-forget from the form handler; the
pendingSubmit guard already prevents re-entry, so it just stays "pending" during
the derivation).

Known gap: hosting the tiny .torrent (xs=) on the edit path needs storage-
credential plumbing the Edit modal doesn't currently have — so the edit path is
ws=-only for now (upload path hosts xs= when storage is unlocked). Tracked as a
follow-up.

Edit-modal submit-state + upload-field tests green. Lint + build green.

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

js/ui/components/EditModal.js

@@ -888,7 +888,7 @@ export class EditModal {
     }
   }
 
-  submit() {
+  async submit() {
     if (this.pendingSubmit) {
       return;
     }
@@ -1076,6 +1076,34 @@ export class EditModal {
       return;
     }
 
+    // Auto-derive a webseed torrent when the URL was changed to an external link
+    // with no magnet, so an edited external URL keeps the P2P benefit (magnet
+    // ws=url). Best-effort + size-capped; on CORS / too-large / any error it stays
+    // URL-only. Runs before the magnet normalization below so the derived magnet's
+    // ws/xs are extracted the same way a pasted magnet's are. (Hosting the tiny
+    // .torrent (xs=) on the edit path needs credential plumbing — a follow-up.)
+    if (
+      urlWasEdited &&
+      finalUrl &&
+      !finalMagnet &&
+      typeof this.mediaUploader?.deriveTorrentForExternalUrl === "function"
+    ) {
+      try {
+        const derived = await this.mediaUploader.deriveTorrentForExternalUrl(
+          finalUrl,
+          {},
+        );
+        if (derived?.magnet) {
+          finalMagnet = derived.magnet;
+        }
+      } catch (error) {
+        devLogger.warn(
+          "[EditModal] External URL webseed derivation skipped:",
+          error?.message || error,
+        );
+      }
+    }
+
     if (finalMagnet) {
       // When the web seed list was manually edited, replace the magnet's full
       // ws= set with exactly what's in the field — this is what makes removal