feat(themes): custom CSS themes per user (upload, paste, live)

- Theme manager in settings: upload a .css file or paste CSS, template
  download/copy/insert, edit and delete; custom themes show up in the
  gallery and the quick switchers
- Themes are injected live as <style id="mindsift-custom-theme">; the
  active one is cached so it applies on boot without flashing
- Backend: user_themes table and GET/POST /themes, DELETE /themes/{id}
  (per user, max 200 KB)
- Docs: theming, REST, database

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/domain/models.go

@@ -449,6 +449,15 @@ type SharedList struct {
 	CreatedAt string           `json:"createdAt"`
 }
 
+// UserTheme is a custom CSS theme of one user.
+type UserTheme struct {
+	ID        string `json:"id"`
+	Name      string `json:"name"`
+	CSS       string `json:"css"`
+	CreatedAt string `json:"createdAt"`
+	UpdatedAt string `json:"updatedAt"`
+}
+
 type APIKey struct {
 	ID         string `json:"id"`
 	UserID     string `json:"userId"`