feat: comprehensive Wino Mail updates

- ReaderView: complete rewrite with CSS styling, headers, attachments, zoom, find, dark mode, image blocking
- Categories: DAO + UI tree widget with colors, nested categories, assignment
- Rules engine: DAO + evaluation + actions (move, mark read, flag, delete, category, forward)
- Templates: DAO + editor + manager with variables support
- Signatures: DAO + manager + auto-per-account + manual selector in compose
- ComposeView: signature combo, template selector, auto-signature on new email
- MainWindow: frameless with rounded corners, 1px border, resize from edges, no status bar
- Database: new tables (Category, MailCategory, Rule, Template, Signature) with indexes

src/db/dao/templatedao.h

@@ -0,0 +1,51 @@
+#pragma once
+
+#include "../databasemanager.h"
+#include <QVector>
+#include <optional>
+#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);
+};
+
+class TemplateDao
+{
+public:
+    static bool insert(Template& tmpl);
+    static bool update(const Template& tmpl);
+    static bool remove(qint64 id);
+    static std::optional<Template> findById(qint64 id);
+    static QVector<Template> findByAccount(qint64 accountId);  // accountId = -1 for global
+    static QVector<Template> findGlobal();
+    static QVector<Template> findAll();
+    static std::optional<Template> findDefault(qint64 accountId);
+    static bool setDefault(qint64 id, bool isDefault);
+};