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/Bold Duotone/Electronic, Devices/Laptop Minimalistic.svg

@@ -0,0 +1,5 @@
+<svg width="24" height="24" viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path fill-rule="evenodd" clip-rule="evenodd" d="M1 20.24C1 19.8202 1.3436 19.48 1.76744 19.48H22.2326C22.6564 19.48 23 19.8202 23 20.24C23 20.6597 22.6564 21 22.2326 21H1.76744C1.3436 21 1 20.6597 1 20.24Z" fill="black"/>
+<path opacity="0.5" d="M3.69013 3.8904C2.79102 4.78079 2.79102 6.21386 2.79102 9.08V14.1467C2.79102 16.0574 2.79102 17.0128 3.39042 17.6064C3.98983 18.2 4.95457 18.2 6.88404 18.2H17.1166C19.0461 18.2 20.0108 18.2 20.6102 17.6064C21.2096 17.0128 21.2096 16.0574 21.2096 14.1467V9.08C21.2096 6.21386 21.2096 4.78079 20.3105 3.8904C19.4114 3 17.9643 3 15.0701 3H8.93055C6.03635 3 4.58924 3 3.69013 3.8904Z" fill="black"/>
+<path d="M8.93053 14.4004C8.50668 14.4004 8.16309 14.7407 8.16309 15.1604C8.16309 15.5801 8.50668 15.9204 8.93053 15.9204H15.0701C15.4939 15.9204 15.8375 15.5801 15.8375 15.1604C15.8375 14.7407 15.4939 14.4004 15.0701 14.4004H8.93053Z" fill="black"/>
+</svg>