Qt desktop: add manage (edit/delete) and tag autocomplete

Brings the Qt uploader to parity with the web /manage page and the Android
app. Adds a stacked Manage page (reached from the post page) that lists
posts via GET /api/photos with thumbnails from GET /api/thumb/<slug>, edits
caption/date/tags in a QDialog via POST /api/edit, and deletes to trash via
POST /api/delete with a QMessageBox confirm. As a non-browser client it sends
no Origin and so passes the server's same-origin CSRF check.

Tag autocomplete now backs every tags field (post cards and the edit dialog)
via a per-token TagCompleter (QCompleter subclass, MatchContains) fed from
the unguarded GET /api/tags.json.

New files: managepostwidget.{h,cpp}, tagcompleter.{h,cpp}; network methods
and a RemotePhoto struct added to uploadclient. Built and linked clean; not
yet runtime-tested on a real display.

CLAUDE.md

@@ -266,7 +266,7 @@ nix build          # succeeds
 ## What's not done yet
 
 - ActivityPub federation (planned later — start with RSS/Atom)
-- Edit/delete in the **Qt desktop** app (web `/manage` and the Android app have it; the `/api/` endpoints are there for Qt to adopt). Restoring from trash is still web-only on all clients.
+- Restoring from trash is still web-only: the `/manage` page has it, but neither the Android nor the Qt desktop app exposes `/api/restore` or `/api/trash` yet. (Edit/delete and tag autocomplete now exist in all three clients.)
 - Any auth beyond the app's htpasswd Basic auth (no sessions, no per-user scopes)
 - The Android app has never been run on real hardware
 - Photo-page swipe has never been tested on a real touchscreen; the iOS Safari edge-gesture interaction is the untested part