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/Messages, Conversation/Multiple Forward Right.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="M12.6644 5.47973L16.6367 9.01066C18.2053 10.405 18.9896 11.1022 18.9896 12.0003C18.9896 12.8984 18.2053 13.5956 16.6367 14.9899L12.6644 18.5209C11.9484 19.1573 11.5903 19.4755 11.2952 19.343C11 19.2104 11 18.7314 11 17.7735V15.4289C7.4 15.4289 3.5 17.1432 2 20.0003C2 10.8574 7.33333 8.57172 11 8.57172V6.22714C11 5.26915 11 4.79015 11.2952 4.6576C11.5903 4.52505 11.9484 4.84327 12.6644 5.47973Z" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+<path opacity="0.5" d="M15.5385 4.5L20.7547 9.34362C21.5488 10.081 22 11.1158 22 12.1994C22 13.3418 21.4988 14.4266 20.6289 15.1671L15.5385 19.5" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+</svg>