fix(starlark): blank app locations use the device location, not San Francisco

A Starlark (Tidbyt) app whose Location field is blank rendered at its
author's hard-coded DEFAULT_LOCATION -- usually San Francisco -- even with
the device city set under General settings. A user in Charlotte, NC got San
Francisco weather and radar with nothing in config.json to explain it.

src/device_location.py fills unset location fields at render time (display
plugin and the web standalone render): the device city is geocoded once via
Open-Meteo, preferring a match in the configured state/country, and cached
permanently. A saved location always wins; if the lookup fails the field is
dropped so the app uses its own default, and the failure is not retried for
30 minutes.

Also fixes the config form: clearing a location omitted the key, and the
save merges, so the old value could never be removed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -25,6 +25,12 @@ accepts both, but the store flags the old spelling as deprecated
   that will now render the font it asked for.
 - `src.wifi_manager.get_wifi_status_path()` — where WiFi status messages for
   the display are written (`config/wifi_status.json`).
+- `src.device_location` — a blank `Location` field on a Starlark (Tidbyt) app
+  now renders at the device's City / State / Country (geocoded once via
+  Open-Meteo and cached) instead of the app author's hard-coded default,
+  usually San Francisco. A location saved on the app still wins. Clearing an
+  app's location in the web UI now actually clears it; the save used to drop
+  the blank field, so the old value stayed.
 
 ## 3.5.0