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/db/migrate.ts

@@ -11,6 +11,7 @@ import { up as up0009ChatReply } from "./migrations/0009_chat_reply.ts";
 import { up as up0010YoutubeEmbedStart } from "./migrations/0010_youtube_embed_start.ts";
 import { up as up0011SplitFaviconThumbnail } from "./migrations/0011_split_favicon_thumbnail.ts";
 import { up as up0012FixFaviconReclassification } from "./migrations/0012_fix_favicon_reclassification.ts";
+import { up as up0013DumpUrlCanonical } from "./migrations/0013_dump_url_canonical.ts";
 
 interface Migration {
   name: string;
@@ -36,6 +37,7 @@ const MIGRATIONS: Migration[] = [
     name: "0012_fix_favicon_reclassification",
     up: up0012FixFaviconReclassification,
   },
+  { name: "0013_dump_url_canonical", up: up0013DumpUrlCanonical },
 ];
 
 export function runMigrations(db: DatabaseSync): void {