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>

docs/STARLARK_APPS_GUIDE.md

@@ -127,7 +127,7 @@ Verify installation:
 3. Filter by category: Weather, Sports, Finance, Games, Clocks, etc.
 4. Click **Install** on desired apps
 5. Configure each app:
-   - Set location/timezone
+   - Set location/timezone (optional: blank uses this device's location)
    - Enter API keys if required
    - Customize display preferences
 
@@ -137,7 +137,11 @@ Each app may have different configuration options:
 
 #### Common Configuration Types
 
-- **Location** (lat/lng/timezone): For weather, clocks, transit
+- **Location** (lat/lng/timezone): For weather, clocks, transit. Left blank,
+  the app renders at this device's location (City / State / Country under
+  General settings). Without that, most community apps fall back to their
+  author's hard-coded default, usually San Francisco. Fill it in only to
+  point one app somewhere else.
 - **API Keys**: For services like weather, stocks, sports scores
 - **Display Preferences**: Colors, units, layouts
 - **Dropdown Options**: Team selections, language, themes