fix: location fallback to constants, compact card sizing

- Add third location fallback to utils/constants (London 51.5074/-0.1278)
  so location always resolves even without GPS or env vars configured
- Remove min-height from sys-card to eliminate wasted space
- Switch System Info to vertical key-value layout filling the card
- Clean up OS string (strip glibc suffix), use locale date for boot time
- Bump info grid font size from 11px to 12px for readability

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

routes/system.py

@@ -452,7 +452,7 @@ def _get_observer_location() -> dict[str, Any]:
             if pos.altitude is not None:
                 gps_meta['altitude'] = round(pos.altitude, 1)
 
-    # Fall back to config defaults
+    # Fall back to config env vars
     if lat is None:
         with contextlib.suppress(Exception):
             from config import DEFAULT_LATITUDE, DEFAULT_LONGITUDE
@@ -460,6 +460,14 @@ def _get_observer_location() -> dict[str, Any]:
             if DEFAULT_LATITUDE != 0.0 or DEFAULT_LONGITUDE != 0.0:
                 lat, lon, source = DEFAULT_LATITUDE, DEFAULT_LONGITUDE, 'config'
 
+    # Fall back to hardcoded constants (London)
+    if lat is None:
+        with contextlib.suppress(Exception):
+            from utils.constants import DEFAULT_LATITUDE as CONST_LAT
+            from utils.constants import DEFAULT_LONGITUDE as CONST_LON
+
+            lat, lon, source = CONST_LAT, CONST_LON, 'default'
+
     result: dict[str, Any] = {'lat': lat, 'lon': lon, 'source': source}
     if gps_meta:
         result['gps'] = gps_meta