fix(config): make the device location the default for plugin location fields (#490)

* fix(config): make the device location the default for plugin location fields

A user in Kansas City reported their radar centred on Dallas, TX with
nothing in config.json to explain it.

The radar is the `ledmatrix-weather` plugin's `radar` mode, and it centres
on the same coordinates as every other weather mode: `forecast_data`
lat/lon, geocoded from the plugin's own `location_city` /
`location_state` / `location_country`. Those ship with schema defaults of
Dallas / Texas / US. A user who never opened the weather plugin's config
form therefore has no `location_city` on disk, and `PluginManager` merges
the schema default in at load time — so the whole plugin (not just the
radar) silently runs on Dallas. Radar is just the only mode that draws a
recognisable map and gives the mismatch away.

Meanwhile the device-wide `location` block that General settings writes
was read by nothing at all, despite its own help text promising it was
"used for weather, sunrise/sunset, and other location-based content".

`SchemaManager.generate_default_config()` now substitutes the device
`location` into the three fully-namespaced `location_*` keys before
handing defaults back, so the promise holds:

- Only `location_city` / `location_state` / `location_country` are
  substituted. A bare `state` key is left alone — `ledmatrix-elections`
  uses it for a two-letter code, and rewriting it would break that plugin.
- A value the user saved on the plugin still wins: this replaces the
  schema default, and `merge_with_defaults` puts user config on top.
- The substitution is applied on the way out of the defaults cache rather
  than into it, so changing the device location takes effect immediately.
- No config manager, no `location` block, or an unreadable config all
  fall back to the plugin's own schema defaults.

Every caller benefits: the plugin loader, the config form (which now
pre-fills the user's real city), config save, and reset-to-defaults.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GNLrSZ32FNKpHRaduKEJsg

* docs(web): name the exact plugin keys the device location seeds

Review follow-up. The General settings help text said the device location
was "the default for every plugin that asks for a city", which overstates
what the code does: only the fully-namespaced `location_city` /
`location_state` / `location_country` keys are substituted. A plugin with
a bare `city` key gets nothing — deliberately, since `ledmatrix-elections`
uses `state` for a two-letter code. The tips now name the exact keys.

Worth noting for anyone editing these: `ui.help_tip(...)` takes a
single-quoted Jinja string, so an apostrophe in the tip text has to be
escaped or written around. The wording here avoids them.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GNLrSZ32FNKpHRaduKEJsg

---------

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

docs/CONFIG_REFERENCE.md

@@ -18,7 +18,7 @@ tooling against it.
 | `web_display_autostart` | bool, `true` | Whether the web interface service starts with the system | `scripts/utils/start_web_conditionally.py` |
 | `timezone` | string, `"America/New_York"` | IANA timezone for schedules and displays | `ConfigManager.get_timezone()` |
 | `target_fps` | int, `100` | Frame-rate ceiling for plugin rendering | `src/plugin_system/base_plugin.py`, `src/common/sports_scroll.py` |
-| `location` | object | `city` / `state` / `country`, offered to plugins that need a location (weather, etc.) | 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. | `SchemaManager.apply_device_location()`, then plugins via merged config |
 
 ## `schedule` — display on/off hours