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.

README.md

@@ -24,7 +24,7 @@ database, no framework.
 
 **Content & management (no database)**
 - `.toml` sidecars are the source of truth; the binary watches the content dir and rebuilds on any change. Four renditions per photo (original, 600² thumbnail, 1600px medium, 1200px OG), each skipped when already up to date.
-- **Web upload** (`/upload`) with EXIF-date prefill and tag autocomplete.
+- **Web upload** (`/upload`): pick one or many photos, each with its own EXIF-prefilled date, caption and tag-autocompleting tags field; posted with per-photo status and retry.
 - **Manage UI** (`/manage`): edit caption/date/tags, bulk tag add/remove, and delete — delete is reversible (moves to `content/.trash/`, with restore / empty).
 - Tag autocomplete across the upload and manage forms, fed by `/tags.json`.