- SettingsView: complete UI with tabs for Accounts, Categories, Rules, Templates, Signatures, General, Appearance
- Categories: QTreeWidget with hierarchical view, color picker, parent categories
- Rules: QListWidget with RuleEditorDialog integration, toggle enabled
- Templates: QListWidget with TemplateEditorDialog integration
- Signatures: QListWidget with SignatureEditorDialog integration
- SignatureDao: full CRUD with default signature support
- common_structs.h: QColor for Category, all structs centralized
- icons.qrc: updated with Linear/Outline/Bold icons from resources
- Category: QColor instead of QString for color
- EmailTreeModel/DateGroupProxyModel: email grouping by date
- MailListView: QTreeView with date grouping combo
- ReaderView: subject frame with shadow, avatar in header, body inside header
- MainWindow: frameless with rounded corners, 1px border, edge resize, no status bar
- icons.qrc: complete icon set from resources/icons/SVG
