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>

web_interface/templates/v3/partials/general.html

@@ -95,7 +95,7 @@
         <!-- Location Information -->
         <div class="grid grid-cols-1 md:grid-cols-3 xl:grid-cols-3 2xl:grid-cols-3 gap-4">
             <div class="form-group" id="setting-general-city" data-setting-key="location.city">
-                <label for="city" class="block text-sm font-medium text-gray-700">City{{ ui.help_tip('City used for weather, sunrise/sunset, and other location-based content.\nExample: Dallas.', 'City') }}</label>
+                <label for="city" class="block text-sm font-medium text-gray-700">City{{ ui.help_tip('City used for weather, sunrise/sunset, radar, and other location-based content.\nExample: Kansas City.\nUsed as the default for the location_city setting on plugins that have one; a value saved on the plugin itself overrides it.', 'City') }}</label>
                 <input type="text"
                        id="city"
                        name="city"
@@ -104,7 +104,7 @@
             </div>
 
             <div class="form-group" id="setting-general-state" data-setting-key="location.state">
-                <label for="state" class="block text-sm font-medium text-gray-700">State{{ ui.help_tip('State or region for your location.\nExample: Texas. Improves location-lookup accuracy.', 'State') }}</label>
+                <label for="state" class="block text-sm font-medium text-gray-700">State{{ ui.help_tip('State or region for your location.\nExample: Missouri. Improves location-lookup accuracy.\nUsed as the default for the location_state setting on plugins that have one.', 'State') }}</label>
                 <input type="text"
                        id="state"
                        name="state"
@@ -113,7 +113,7 @@
             </div>
 
             <div class="form-group" id="setting-general-country" data-setting-key="location.country">
-                <label for="country" class="block text-sm font-medium text-gray-700">Country{{ ui.help_tip('Country code or name for your location.\nExample: US. Used with City and State for weather and geolocation.', 'Country') }}</label>
+                <label for="country" class="block text-sm font-medium text-gray-700">Country{{ ui.help_tip('Country code or name for your location.\nExample: US. Used with City and State for weather, radar, and geolocation.\nUsed as the default for the location_country setting on plugins that have one.', 'Country') }}</label>
                 <input type="text"
                        id="country"
                        name="country"