feat(theming): settings-store, nav merge util and radius tokens

Phases 0-2 of docs/website/THEMING_AND_NAV.md. Groundwork only: no admin UI,
no consumer wiring, and an instance that never touches the new settings keys
renders exactly as it does today.

Phase 0 - settings store:
- settingsDb.remove() and DELETE /api/v1/admin/settings/:key, the "reset to
  default" primitive. Defaults for these keys live in BRAND_* env, theme.css
  and the hardcoded NAV arrays, so reset has to delete the row rather than
  store a copy of the default. Allowlisted to the five theming/nav keys plus
  hero_layout_draft, admin-only, idempotent.
- GET /api/v1/settings/nav behind requireAuth with no role gate. AdminLayout
  renders for editors and moderators and PlayerPortalLayout for players, and
  none of them can read GET /admin/settings, so without this their nav
  override would silently never apply.
- A fifth router group for it: /public is anonymous, /admin/settings is
  adminOnly, /player is self-scoped data. This is configuration that needs a
  login.
- parseJsonSetting() in utils/settingsJson.js. settings.value is TEXT, so
  every JSON key arrives as a string; malformed or wrong-shaped reads as
  absent, never as an error and never half-applied.
- theme_visual / brand_assets / nav_public join PUBLIC_KEYS; nav_admin and
  nav_player deliberately do not.

Phase 1 - client/src/lib/navOverrides.js, the pure merge util. Presentation
only: it can set label/order/hidden and (grouped navs) group, and nothing
else. It cannot introduce a `to`, cannot touch roles/feature, and hidden:false
cannot un-hide anything - the existing filters run afterward, unchanged, and
remain the boundary.

Phase 2 - promoted 23 border-radius literals in theme.css to four tokens at
today's values (14x8px, 4x999px, 4x10px, 1x12px). The 7px/6px editor chrome
and the two 50% circles stay literal. --shadow-card and --panel-grad were
already tokens.

Tests: 16 new server tests, 20 new client tests. The route-manifest guard now
also asserts /settings/** sits behind requireAuth. Swagger and both route
artifacts regenerated.

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

server/src/model/settings/settings.db.js

@@ -22,4 +22,12 @@ async function seedDefault(key, value) {
   await query('INSERT IGNORE INTO settings (`key`, value) VALUES (?, ?)', [key, value])
 }
 
-module.exports = { getAll, get, set, seedDefault }
+// Delete a settings row. "Reset to defaults" for the theming/nav keys is the
+// *absence* of a row, not a stored copy of the defaults — see
+// docs/website/THEMING_AND_NAV.md §2. Deleting a key that was never set is a
+// no-op, so reset is idempotent.
+async function remove(key) {
+  await query('DELETE FROM settings WHERE `key` = ?', [key])
+}
+
+module.exports = { getAll, get, set, seedDefault, remove }