Fix/sdk default (#2199)

* feat(plugin-sdk): settings-field default + unknown-attribute warning

* feat(shared): default on the plugin settings-field contract

* feat(server): persist and serve plugin settings-field defaults

* feat(client): pre-fill plugin settings defaults in the admin and user forms

* feat(server): enforce required plugin settings on both save paths

* feat(client): block plugin-settings save on an empty required field

* feat(server): resolve plugin settings defaults at runtime

A settings-field `default` was stored and served to the form but never
reached the plugin: the child's init envelope, `ctx.settings.get()`, the
channel dispatch's readAll and the OAuth provider config all read the
stored blob only. Fold `stored ?? default` in on every path that hands
config to a plugin (settings-defaults.ts), and let a default satisfy
`required` on both save gates — refusing a save the runtime would have
accepted was a contradiction.

Also: one `isFilled` rule shared by the save gate and `hasRequired`
(checkbox exempt, whitespace empty); the install parser drops a
non-boolean checkbox default or a select default outside its options,
matching what the SDK already refuses; export SETTING_FIELD_KEYS so the
SDK's mirror can be parity-tested.

* feat(client): share the plugin settings form rules across all three forms

The mobile user settings screen was a third consumer of the fields/config
contract that never got `default` pre-fill or the required check, and
the seed/patch loops lived in five hand copies. Extract
seedSettingsValues / findMissingRequired / settingsPatch into one module
and use it from the admin instance form, the desktop user tab and the
mobile screen. The user forms now surface a 4xx `{ error }` from the
server (the required-field refusal) instead of the generic toast.

* feat(plugin-sdk): seed dev fixtures from manifest defaults

`trek-plugin dev` handed the plugin only the fixture values, so a plugin
relying on its manifest defaults read undefined in dev and only in dev.
Seed ctx.config / ctx.settings from the declared defaults (a fixture
value still wins) via a new exported settingDefaults(manifest, scope).
SETTING_FIELD_KEYS is now parity-tested against the host's list.

* docs(plugins): document settings defaults as runtime fallbacks

A `default` is the field's effective value wherever nobody set one, not
just a form pre-fill; `required` is enforced on both ends; unknown
attributes are dropped at install and warned on by validate. Add a
Settings section with the attribute table to the SDK README and update
the ctx type comments, the shared schema, and the wiki guide.

* add missing exports

shared/src/i18n/pl/admin.ts

@@ -447,6 +447,7 @@ const admin: TranslationStrings = {
   'admin.plugins.instanceSettings': 'Ustawienia instancji',
   'admin.plugins.settingsSaved': 'Ustawienia zapisane',
   'admin.plugins.settingsSavedRestarted': 'Ustawienia zapisane — wtyczka została uruchomiona ponownie',
+  'admin.plugins.requiredMissing': '"{field}" jest wymagane',
   'admin.plugins.cap.readsTrips': 'Odczytuje Twoje podróże',
   'admin.plugins.cap.readsUsers': 'Odczytuje podstawowe profile',
   'admin.plugins.cap.readsCosts': 'Odczytuje Twoje wydatki',