feat(core,fuse,gui): Proton theme, photos on the map and an opt-in street map

- The Proton accent preference becomes a whole Proton theme that also
  holds against a GTK theme imported from gtk.css; folders get Proton's
  purple folder icon while it is on.
- Places map markers show one of each town's photos with a count, and
  zoomed in on a town every photo sits where it was taken
  (Request::PlaceLocations). Photos taken on one spot are listed in a
  popover. "Show on map" in the photo viewer goes to the Places map.
- The built-in map draws country borders, sea and land in their own
  colours.
- An opt-in street map from OpenFreeMap (vector tiles drawn with cairo,
  cached on disk up to 256 MB), with the OpenStreetMap credit on the map
  and in About.
- Translations for all catalogs; changelog folded into 2.3.0.

README.md

@@ -75,7 +75,7 @@ pdfs-app            # the desktop app
 - **File Version History**: Every revision Proton Drive still holds for a file, from the browser's details pane (**Versions**) or `pdfs versions list|restore|save|rm`. Restoring is server-side — no re-upload — and an old version can be written out to a local file without touching the live one.
 - **Photo Favourites**: Star a photo in the lightbox, filter the gallery to favourites, or use `pdfs favorite <uid>` / `pdfs photos --favorites`.
 - **Photo Albums**: Browse your albums — including the ones shared with you — from the Albums view of the Photos page, or with `pdfs albums` / `pdfs album <uid>`. Album contents open in the same gallery as the timeline.
-- **Places**: The Places view of the Photos page groups photos by the town they were taken in, using the location stored with each photo, as cards or on a world map. Towns are looked up offline in [GeoNames](https://www.geonames.org/) data (CC BY 4.0), and the map is drawn offline from [Natural Earth](https://www.naturalearthdata.com/) land outlines (public domain), so no map tiles are fetched; install `iso-codes` for country names in your language.
+- **Places**: The Places view of the Photos page groups photos by the town they were taken in, using the location stored with each photo, as cards or on a world map. Towns are looked up offline in [GeoNames](https://www.geonames.org/) data (CC BY 4.0), and the map is drawn offline from [Natural Earth](https://www.naturalearthdata.com/) land outlines and country borders (public domain). Each town is marked with one of its photos, and zoomed in, every photo sits where it was taken. A street map from [OpenFreeMap](https://openfreemap.org/) (© [OpenStreetMap](https://www.openstreetmap.org/copyright) contributors) can be turned on under Appearance; it is off by default because the tile server learns which parts of the world you look at. Install `iso-codes` for country names in your language.
 - **Human Verification (CAPTCHA) Recovery**: Detects sign-in gates (VPN/new IP challenges) and launches an embedded `WebKitWebView` dialog to safely complete the challenge, transparently retrying authentication with the earned token.
 - **Selective Sync (`.pdfsignore`)**: Keep build trees, dependency directories, and editor leftovers out of synced folders using gitignore-style rules.
 - **Data-Safe Offline Writes**: Durable scratch/staging files and a transactional pending queue preserve acknowledged writes across network failures and restarts.