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

resources/icons/SVG/Broken/Notifications/Notification Unread Lines.svg

@@ -0,0 +1,6 @@
+<svg width="24" height="24" viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg">
+<circle cx="19" cy="5" r="3" stroke="black" stroke-width="1.5"/>
+<path d="M7 14H16" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+<path d="M7 17.5H13" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+<path d="M2 12C2 16.714 2 19.0711 3.46447 20.5355C4.92893 22 7.28595 22 12 22C16.714 22 19.0711 22 20.5355 20.5355C22 19.0711 22 16.714 22 12V10.5M13.5 2H12C7.28595 2 4.92893 2 3.46447 3.46447C2.49073 4.43821 2.16444 5.80655 2.0551 8" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+</svg>