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/Nature, Travel/Suitcase 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">
+<path d="M16 6C16 4.11438 16 3.17157 15.4142 2.58579C14.8284 2 13.8856 2 12 2C10.1144 2 9.17157 2 8.58579 2.58579C8 3.17157 8 4.11438 8 6" stroke="black" stroke-width="1.5"/>
+<path d="M2.10913 11L22 11" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+<path d="M22 17L18.5 17M2.5 17L13.5 17" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+<path d="M10 22C6.22876 22 4.34315 22 3.17157 20.8284C2 19.6569 2 17.7712 2 14C2 10.2288 2 8.34315 3.17157 7.17157C4.34315 6 6.22876 6 10 6M14 22C17.7712 22 19.6569 22 20.8284 20.8284C22 19.6569 22 17.7712 22 14C22 10.2288 22 8.34315 20.8284 7.17157C19.6569 6 17.7712 6 14 6" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+</svg>