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/handlers/admin.go

@@ -369,7 +369,6 @@ func (h *AdminHandler) PublicSettings(c *gin.Context) {
 		"storage_configured": storageConfigured,
 		"policies": gin.H{
 			"allow_registration": policies["allow_registration"],
-			"allow_user_byok":    policies["allow_user_byok"],
 		},
 	})
 }