feat(auth): GetProfile returns the account theme

The Android app's app-chrome theme (UX pass item 6) reads the account
theme from GET /api/auth/profile and re-applies it at startup in
SERVER sync mode. PUT /api/auth/theme already persisted the value;
this completes the round trip on the profile read.

internal/handlers/auth.go

@@ -101,6 +101,7 @@ type UserProfile struct {
 	FirstName string `json:"first_name"`
 	LastName  string `json:"last_name"`
 	Role      string `json:"role"`
+	Theme     string `json:"theme,omitempty"`
 }
 
 type UpdateProfileRequest struct {
@@ -502,6 +503,7 @@ func (h *AuthHandler) GetProfile(c *echo.Context) error {
 		FirstName: firstName,
 		LastName:  lastName,
 		Role:      user.Role,
+		Theme:     user.Theme.String,
 	})
 }