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/Linear/Users/User Hand Up.svg

@@ -0,0 +1,4 @@
+<svg width="24" height="24" viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M8 13H16M8 13V18C8 19.8856 8 20.8284 8.58579 21.4142C9.17157 22 10.1144 22 12 22C13.8856 22 14.8284 22 15.4142 21.4142C16 20.8284 16 19.8856 16 18V13M8 13C5.2421 12.3871 3.06717 10.2687 2.38197 7.52787L2 6M16 13C17.7107 13 19.1506 14.2804 19.3505 15.9795L20 21.5" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+<circle cx="12" cy="6" r="4" stroke="black" stroke-width="1.5"/>
+</svg>