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/Security/Key Minimalistic 2.svg

@@ -0,0 +1,6 @@
+<svg width="24" height="24" viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M21.0635 12.5C21.6591 11.4704 22 10.275 22 9C22 5.13401 18.866 2 15 2C11.134 2 8 5.13401 8 9C8 12.866 11.134 16 15 16C16.0736 16 17.0907 15.7583 18 15.3264" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+<circle cx="15" cy="9" r="2" stroke="black" stroke-width="1.5"/>
+<path d="M3.5 20.5L9.5 14.5" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+<path d="M6 21L4.5 19.5M6.5 17.5L8 19" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+</svg>