[client] Add light mode with system, light, and dark theme options (#7344)

* desktop UI light mode

* Theme review fixes plus macOS window outline fix

* Windows runtime chrome re-theming plus apply serialization

* Windows chrome threading and theme event ordering fixes

* Darken toggle and setting sidebar text

* resolve theme appearance, apply on UI thread

* read theme once per window

* Re-assert Windows dark opt-in after SetTheme

* split app-wide GTK theming from per-window chrome

* Update Wails dependency and checksums

* KDE tray icon panel fix

* Five review fixes: theme ordering, cgo dedup, KDE panel resolution

* Path guard hardening, toggle contrast, windows comment

* non-vacuous escape tests

* Default view edits

* Polish settings nav, controls, borders, and disc

* Profiles settings boarder, modals, and buttons

* Additional edits based on feedback

* Switch colors away from slight blue hue

* Update missing lang

* Fix vertical tab active view

client/ui/preferences/store.go

@@ -49,10 +49,34 @@ func (v ViewMode) IsValid() bool {
 	return false
 }
 
+// Theme is the preferred UI appearance: follow the OS ("system") or force
+// "light"/"dark".
+type Theme string
+
+const (
+	ThemeSystem Theme = "system"
+	ThemeLight  Theme = "light"
+	ThemeDark   Theme = "dark"
+)
+
+// DefaultTheme applies when no file exists or its theme is empty/unknown.
+const DefaultTheme = ThemeSystem
+
+var ErrUnsupportedTheme = errors.New("unsupported theme")
+
+func (t Theme) IsValid() bool {
+	switch t {
+	case ThemeSystem, ThemeLight, ThemeDark:
+		return true
+	}
+	return false
+}
+
 // UIPreferences is rewritten in full on every change; there are no partial updates.
 type UIPreferences struct {
 	Language            i18n.LanguageCode `json:"language"`
 	ViewMode            ViewMode          `json:"viewMode"`
+	Theme               Theme             `json:"theme"`
 	OnboardingCompleted bool              `json:"onboardingCompleted"`
 	// AutostartInitialized records that the one-time autostart default
 	// decision has run for this OS user. It only ever transitions to true
@@ -105,7 +129,7 @@ func NewStore(validator LanguageValidator, emitter Emitter) (*Store, error) {
 		path:      path,
 		validator: validator,
 		emitter:   emitter,
-		current:   UIPreferences{ViewMode: DefaultViewMode},
+		current:   UIPreferences{ViewMode: DefaultViewMode, Theme: DefaultTheme},
 	}
 
 	if err := s.load(); err != nil {
@@ -146,6 +170,30 @@ func (s *Store) SetViewMode(mode ViewMode) error {
 	return nil
 }
 
+// SetTheme validates, persists, and broadcasts. No-op if unchanged.
+func (s *Store) SetTheme(theme Theme) error {
+	if !theme.IsValid() {
+		return fmt.Errorf("%w: %q", ErrUnsupportedTheme, theme)
+	}
+
+	s.mu.Lock()
+	if s.current.Theme == theme {
+		s.mu.Unlock()
+		return nil
+	}
+	next := s.current
+	next.Theme = theme
+	if err := s.persistLocked(next); err != nil {
+		s.mu.Unlock()
+		return fmt.Errorf("persist preferences: %w", err)
+	}
+	s.current = next
+	s.mu.Unlock()
+
+	s.broadcast(next)
+	return nil
+}
+
 // SetOnboardingCompleted persists the welcome-window dismissal. No-op if unchanged.
 func (s *Store) SetOnboardingCompleted(done bool) error {
 	s.mu.Lock()
@@ -288,6 +336,9 @@ func (s *Store) load() error {
 	if !loaded.ViewMode.IsValid() {
 		loaded.ViewMode = DefaultViewMode
 	}
+	if !loaded.Theme.IsValid() {
+		loaded.Theme = DefaultTheme
+	}
 
 	s.mu.Lock()
 	s.current = loaded