fix(starlark): blank app locations use the device location, not San Francisco (#617)

* 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): say what happens when the device location can't be used

A blank app Location only renders at the device's city when one is set and
the Open-Meteo lookup finds it. With no city, no match, or the geocoder
unreachable (retried after 30 minutes), the app gets no location and keeps
its author's default. The guide, the config page hint, CONFIG_REFERENCE and
the CHANGELOG entry now say so.

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

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

docs/CONFIG_REFERENCE.md

@@ -19,7 +19,7 @@ tooling against it.
 | `auto_update.enabled` | bool, `false` | Weekly automatic updates: LEDMatrix code first (health-checked, rolled back on failure), then installed plugins. Toggle in the General tab or install with `first_time_install.sh --enable-auto-update` | `web_interface/auto_update.py`, `src/auto_update_setup.py` (`is_enabled()`) |
 | `timezone` | string, `"America/New_York"` | IANA timezone for schedules and displays | `ConfigManager.get_timezone()` |
 | `target_fps` | int, `100` | Legacy "Scroll Frame Rate". Core scrolling no longer reads it: scroll frames are presented at `display.hardware.limit_refresh_rate_hz` divided by each scroll's frame hold, and speed comes from each plugin's scroll settings. Still exposed to plugins via `BasePlugin.global_config` | `src/plugin_system/base_plugin.py` |
-| `location` | object | `city` / `state` / `country`. Supplies the **default** for a plugin's own `location_city` / `location_state` / `location_country` setting, so weather, radar and friends follow this device without being configured twice. A value saved on the plugin itself still overrides it. | `SchemaManager.apply_device_location()`, then plugins via merged config |
+| `location` | object | `city` / `state` / `country`. Supplies the **default** for a plugin's own `location_city` / `location_state` / `location_country` setting, so weather, radar and friends follow this device without being configured twice. A value saved on the plugin itself still overrides it. Starlark (Tidbyt) apps get the same treatment: a `Location` field left blank on the app renders at this city (geocoded once via Open-Meteo, coordinates cached permanently) instead of the app author's default, which is usually San Francisco. If the city can't be looked up (no match, or the geocoder is unreachable; retried after 30 minutes), the app keeps its own default. | `SchemaManager.apply_device_location()`, then plugins via merged config; `src/device_location.py` for Starlark apps |
 
 ## `schedule` — display on/off hours
 

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 +137,12 @@ 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). If no city is set there, or the city can't be looked up
+  (no match, or the geocoder is unreachable -- retried after 30 minutes), the
+  app gets no location and falls back to its 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