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/Line Duotone/Shopping, Ecommerce/Cart 4.svg

@@ -0,0 +1,7 @@
+<svg width="24" height="24" viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M3.55514 14.2572C2.83668 10.9043 2.47745 9.22793 3.378 8.11397C4.27855 7 5.99302 7 9.42196 7H14.5781C18.0071 7 19.7215 7 20.6221 8.11397C21.5226 9.22793 21.1634 10.9043 20.4449 14.2572L20.0164 16.2572C19.5294 18.5297 19.2859 19.666 18.4608 20.333C17.6357 21 16.4737 21 14.1495 21H9.85053C7.52639 21 6.36432 21 5.53925 20.333C4.71418 19.666 4.47069 18.5297 3.98372 16.2572L3.55514 14.2572Z" stroke="black" stroke-width="1.5"/>
+<path opacity="0.6" d="M3 11H21" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+<path opacity="0.6" d="M10 14H14" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+<path opacity="0.6" d="M18 9L15 3" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+<path opacity="0.6" d="M6 9L9 3" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+</svg>