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

src/model.ts

@@ -74,6 +74,32 @@ export interface Dump {
 
 export type RawDump = WithStringDate<Dump>;
 
+/**
+ * `GET /api/preview`. `reached` is false when the page could not be fetched, in
+ * which case `richContent` is a hostname-only stub rather than real metadata.
+ */
+export interface UrlPreviewResponse {
+  reached: boolean;
+  richContent: RichContent | null;
+}
+
+/** An existing dump on the same URL, as returned by `GET /api/dumps/by-url`. */
+export interface DumpUrlMatch {
+  id: string;
+  slug?: string;
+  title: string;
+  username: string;
+  createdAt: Date;
+  voteCount: number;
+  commentCount: number;
+}
+
+export type RawDumpUrlMatch = WithStringDate<DumpUrlMatch>;
+
+export function deserializeDumpUrlMatch(raw: RawDumpUrlMatch): DumpUrlMatch {
+  return { ...raw, createdAt: new Date(raw.createdAt) };
+}
+
 export function deserializeDump(raw: RawDump): Dump {
   return {
     ...raw,
@@ -671,6 +697,8 @@ export interface RegisterRequest {
 
 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[];