Feature: Settings screen with account, theme, profile, reset + credits

Backend:
- New GET/PUT /api/me endpoint to read and update the authenticated
  user's profile (currently display_name). UserStore.UpdateDisplayName.
- Wired into the authed router.

App:
- New SettingsScreen + SettingsViewModel with five sections:
  * Account & Sync: login status, email, server URL, logout, connect.
  * Profile: edit display name (pushed to PUT /api/me when logged in).
  * Appearance: System / Light / Dark theme switch, persisted in
    SessionManager and applied via MitbringslTheme(sessionManager).
  * Data: 'Reset local data' wipes Room tables (server data kept).
  * About: version, 'Developed by Janik Dietz', and credits to
    GLM-5.2 + Gemini 3.6 Flash.
- Theme.kt now reads the user's theme preference (StateFlow) instead
  of only the system default; MainActivity passes SessionManager in.
- Navigation: new SettingsNavKey; settings gear icon in ListsScreen
  top bar; Settings links back to the Sync/Account screen.
- DTOs/API: UpdateMeRequestDto + getMe()/updateMe() for /api/me.

backend/internal/httpapi/api.go

@@ -21,6 +21,7 @@ type API struct {
 	ops     *OpsHandler
 	suggest *SuggestHandler
 	config  *ConfigHandler
+	me      *MeHandler
 }
 
 // NewAPI constructs the API with all handler groups.
@@ -51,6 +52,7 @@ func NewAPI(cfg *config.Config, pool *pgxpool.Pool) *API {
 		ops:     NewOpsHandler(opStore, listStore),
 		suggest: NewSuggestHandler(suggestStore),
 		config:  NewConfigHandler(cfg),
+		me:      NewMeHandler(users),
 	}
 }
 
@@ -85,6 +87,8 @@ func (a *API) Handler() http.Handler {
 	mux.HandleFunc("POST /auth/logout", a.auth.Logout)
 
 	// --- authenticated API endpoints ---
+	mux.Handle("GET /api/me",             a.RequireAuth(http.HandlerFunc(a.me.Get)))
+	mux.Handle("PUT /api/me",             a.RequireAuth(http.HandlerFunc(a.me.Update)))
 	mux.Handle("GET /api/lists",           a.RequireAuth(http.HandlerFunc(a.lists.List)))
 	mux.Handle("POST /api/lists",          a.RequireAuth(http.HandlerFunc(a.lists.Create)))
 	mux.Handle("POST /api/lists/join",     a.RequireAuth(http.HandlerFunc(a.lists.Join)))