feat: complete settings view + signatures + signatures DAO + icons

- 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

src/db/dao/templatedao.h

@@ -6,35 +6,7 @@
 #include <QJsonDocument>
 #include <QJsonObject>
 #include <QJsonArray>
-
-struct TemplateVariable
-{
-    QString name;
-    QString label;
-    QString defaultValue;
-    QString type;  // "text", "email", "date", "select", "textarea"
-    QStringList options;  // for select type
-};
-
-struct Template
-{
-    qint64 id{-1};
-    qint64 accountId{-1};  // -1 = global
-    QString name;
-    QString subject;
-    QString bodyHtml;
-    QString bodyText;
-    QVector<TemplateVariable> variables;
-    bool isDefault{false};
-    QDateTime createdAt;
-    QDateTime updatedAt;
-
-    bool isGlobal() const { return accountId < 0; }
-    bool isValid() const { return id >= 0 && !name.isEmpty(); }
-    
-    QJsonObject toJson() const;
-    static Template fromJson(const QJsonObject& obj);
-};
+#include "common_structs.h"
 
 class TemplateDao
 {