v3: reworked posting — three panels, drafts, duplicate detection, upload progress

Posting a dump was a single form where the important choices were the easiest
to miss. It is now three panels: link or file, why & where, playlists.

Composition:
- No more URL/File toggle. An empty panel offers both ways in at once and the
  kind follows what you actually did; a file dropped anywhere in the modal is
  accepted, not just on the zone.
- Categories and visibility get their own panel instead of a disclosure that
  read as optional, and the primary button stays "Next" until they've been
  seen. Visibility carries a real label now.
- The draft (link, title, why, categories, visibility) is mirrored to
  localStorage on every change and restored on reopen, so Escape or a stray
  backdrop click costs nothing. Only an attached file can't be restored, so
  that is the one case that asks before closing.
- URL dumps can carry a poster-supplied title instead of being stuck with
  whatever the page scraped, editable right under the preview.
- Multipart uploads go through XHR so there is a real progress bar and a
  percentage on the button, rather than 50 MB of silence.

Duplicates:
- New dumps.url_canonical column (+ index, backfilled by 0013) holding a lossy
  key that ignores scheme, www., trailing slashes, tracking parameters and
  YouTube share shapes. GET /api/dumps/by-url reads it, and the create form
  warns "already dumped by X" while it fetches the preview. Never blocking.

Fixes:
- /api/preview now reports whether the page was actually reached: a failed
  fetch still yields a hostname-only stub, so a dead link and a page without
  metadata used to render identically.
- The Web Share Target never worked. The manifest posts to "/", but the index
  redirect dropped the query string, so every Android share landed on the feed
  with nothing pre-filled.
- File dumps no longer take the extension into their title.
- The link field no longer autofocuses on touch, where it raised a keyboard
  over the modal.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TiAPtJZeCYYk8rKehtLUQU

api/model/interfaces.ts

@@ -479,11 +479,27 @@ function isStringArray(value: unknown): value is string[] {
 
 export interface CreateUrlDumpRequest {
   url: string;
+  /** Overrides the title scraped from the page — the poster edited the preview. */
+  title?: string;
   comment?: string;
   isPrivate?: boolean;
   categoryIds?: string[];
 }
 
+/**
+ * An existing dump pointing at the same URL, as shown by the create form's
+ * duplicate hint. Display-only projection — see `findDumpsByUrl`.
+ */
+export interface DumpUrlMatch {
+  id: string;
+  slug?: string;
+  title: string;
+  username: string;
+  createdAt: Date;
+  voteCount: number;
+  commentCount: number;
+}
+
 export function isCreateUrlDumpRequest(
   obj: unknown,
 ): obj is CreateUrlDumpRequest {
@@ -499,6 +515,13 @@ export function isCreateUrlDumpRequest(
     typeof o.comment === "string" &&
     (o.comment as string).length > VALIDATION.DUMP_COMMENT_MAX
   ) return false;
+  if ("title" in o && typeof o.title !== "string" && o.title !== null) {
+    return false;
+  }
+  if (
+    typeof o.title === "string" &&
+    (o.title as string).length > VALIDATION.DUMP_TITLE_MAX
+  ) return false;
   if ("isPrivate" in o && typeof o.isPrivate !== "boolean") return false;
   if ("categoryIds" in o && !isStringArray(o.categoryIds)) return false;
   return true;