Add bulk (multi-photo) upload to the web form

The /upload page now takes several photos at once, mirroring the Android and Qt
apps: a card per photo with its own preview, EXIF-prefilled date, caption and
tag-autocompleting tags field, a remove button, and per-photo status. "Post
photos" uploads them sequentially with an x/N progress line; failures can be
retried by pressing Post again, successes link straight to the new photo page.

No server change — each photo is still a separate POST /upload, exactly as the
apps' per-photo queue does it, so slugs come from filenames as before.

CLAUDE.md

@@ -19,7 +19,9 @@ The binary does three things at once:
 - Serves a multipart upload API at `POST /upload`
 - In dev mode (`--serve`), also serves `public/` as static files
 
-Routes: `GET /upload` (HTML upload form), `POST /upload` (multipart),
+Routes: `GET /upload` (HTML upload form — multi-photo: the page posts one
+`POST /upload` per selected photo, each with its own caption/date/tags),
+`POST /upload` (multipart, one image per request),
 `GET /manage` (edit/delete UI), the manage API under `/api/` (see Manage),
 `GET /health`. Every route except `/health` is gated by HTTP Basic auth when
 `-htpasswd` is set (see Auth).