Feat v0.2.7 user settings audit (#11)

Audit all 6 user settings sections. Remove dead chat-era code,
rename localStorage namespace, clean up stale backend policies.

- Remove BYOK nav section (empty BYOK_ITEMS, byokEnabled state,
  "BYOK Enabled" badge) and personas gate filter from settings nav
- Remove auth.permissions.changed listener (existed solely for
  dead BYOK + personas state)
- Remove Message Font Size slider, msgFont state, and --msg-font
  CSS variable application from appearance section and base template
- Rename localStorage key cs-appearance → sb-appearance with
  one-time migration preserving existing user preferences
- Remove allow_user_byok and allow_user_personas from PolicyDefaults,
  profile bootstrap, permissions handler, PublicSettings, and test seeds
- Remove orphaned .settings-nav-footer CSS class
- Replace stale policy-gating test assertions with allow_registration

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/database/testhelper.go

@@ -340,8 +340,6 @@ func TruncateAll(t *testing.T) {
 		`)
 		DB.Exec(`
 			INSERT INTO platform_policies (key, value) VALUES
-				('allow_user_byok',         'false'),
-				('allow_user_personas',     'false'),
 				('allow_registration',      'true'),
 				('default_user_active',     'false'),
 				('allow_team_providers',    'true')
@@ -357,8 +355,6 @@ func TruncateAll(t *testing.T) {
 		`)
 		DB.Exec(`
 			INSERT INTO platform_policies (key, value) VALUES
-				('allow_user_byok',         'false'),
-				('allow_user_personas',     'false'),
 				('allow_registration',      'true'),
 				('default_user_active',     'false'),
 				('allow_team_providers',    'true')