Remove Immich URL/API key from the config page

Now that they're set via docker-compose.yml's environment (previous
commit), leaving editable fields for them on the page was misleading --
anything typed there would be silently overwritten by the env vars on the
next load() anyway. Replaced with a read-only info banner showing the
configured Immich URL (never the API key value, even though it's already
env-sourced rather than user input) or a warning if IMMICH_URL/
IMMICH_API_KEY aren't set. POST /api/config no longer accepts or touches
those two fields at all.

Verified: page renders with no immich_url/immich_api_key input fields or
API key value in either case (env vars set or unset); config save/albums/
frame-image still work end-to-end via a mock Immich server.

server/app/main.py

@@ -61,16 +61,16 @@ def api_albums():
 
 @app.post("/api/config")
 def api_config_save(
-    immich_url: str = Form(""),
-    immich_api_key: str = Form(""),
     album_id: str = Form(""),
     order: str = Form("sequential"),
     refresh_interval_s: int = Form(3600),
     smart_crop_faces: bool = Form(True),
 ):
+    # Immich URL/API key are env-var only (IMMICH_URL/IMMICH_API_KEY, see
+    # docker-compose.yml.example) -- config.load() already applies them,
+    # and this handler doesn't touch cfg.immich_url/immich_api_key at all,
+    # so there's nothing here that could overwrite or clear them.
     cfg = config.load()
-    cfg.immich_url = immich_url.strip()
-    cfg.immich_api_key = immich_api_key.strip()
     if album_id != cfg.album_id:
         cfg.cursor = 0  # restart from the top of a newly selected album
     cfg.album_id = album_id