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/Bold/Network, IT, Programming/Slash Circle.svg

@@ -0,0 +1,3 @@
+<svg width="24" height="24" viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path fill-rule="evenodd" clip-rule="evenodd" d="M12 22C17.5228 22 22 17.5228 22 12C22 6.47715 17.5228 2 12 2C6.47715 2 2 6.47715 2 12C2 17.5228 6.47715 22 12 22ZM14.0184 7.3645C14.1257 6.9644 13.8882 6.55315 13.4881 6.44595C13.088 6.33874 12.6768 6.57618 12.5696 6.97628L9.98137 16.6355C9.87416 17.0356 10.1116 17.4469 10.5117 17.5541C10.9118 17.6613 11.323 17.4239 11.4303 17.0238L14.0184 7.3645Z" fill="black"/>
+</svg>