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/services/mailservice.cpp

@@ -406,13 +406,13 @@ void MailService::fetchMails(const QString &accountId, const QString &folderId)
     QObject::connect(sync, SIGNAL(progressChanged(int)), this, SIGNAL(progressChanged(int)), Qt::QueuedConnection);
     QObject::connect(sync, SIGNAL(statusMessage(const QString&)), this, SIGNAL(statusMessage(const QString&)), Qt::QueuedConnection);
 
-    QFuture<QVector<MailItem>> future = QtConcurrent::run([=]() {
+    QFuture<QVector<MailItem>> future = QtConcurrent::run([this, sync, folderId]() {
         // SinceUid = 0 for full sync; could be stored per folder but omitted for simplicity
         return sync->fetchMailItems(folderId, 0);
     });
 
     QFutureWatcher<QVector<MailItem>> *watcher = new QFutureWatcher<QVector<MailItem>>(this);
-    QObject::connect(watcher, &QFutureWatcher<QVector<MailItem>>::finished, this, [=]() {
+    QObject::connect(watcher, &QFutureWatcher<QVector<MailItem>>::finished, this, [this, sync, watcher, accountId, folderId]() {
         // Disconnect progress signals
         QObject::disconnect(sync, SIGNAL(progressChanged(int)), this, SIGNAL(progressChanged(int)));
         QObject::disconnect(sync, SIGNAL(statusMessage(const QString&)), this, SIGNAL(statusMessage(const QString&)));
@@ -504,4 +504,3 @@ void MailService::markAsRead(const QString &mailItemId, bool read)
         emit mailReadStateChanged(mailItemId, read);
 }
 
-#include "mailservice.moc"