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/Messages, Conversation/Multiple Forward Left.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="M11.3356 5.47973L7.36335 9.01066C5.79473 10.405 5.01041 11.1022 5.01041 12.0003C5.01041 12.8984 5.79472 13.5956 7.36335 14.9899L11.3356 18.5209C12.0517 19.1573 12.4097 19.4755 12.7048 19.343C13 19.2104 13 18.7314 13 17.7735V15.4289C16.6 15.4289 20.5 17.1432 22 20.0003C22 10.8574 16.6667 8.57172 13 8.57172V6.22714C13 5.26915 13 4.79015 12.7048 4.6576C12.4097 4.52505 12.0517 4.84327 11.3356 5.47973Z" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M8.46132 4.5L3.24512 9.34362C2.45101 10.081 1.99979 11.1158 1.99979 12.1994C1.99979 13.3418 2.50101 14.4266 3.3709 15.1671L8.46132 19.5" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+</svg>