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/Video, Audio, Sound/Vinyl Record.svg

@@ -0,0 +1,5 @@
+<svg width="24" height="24" viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg">
+<circle cx="12" cy="12" r="3" stroke="black" stroke-width="1.5"/>
+<path d="M4.92893 19.0711C8.83418 22.9763 15.1658 22.9763 19.0711 19.0711C22.9763 15.1658 22.9763 8.83418 19.0711 4.92893C15.1658 1.02369 8.83418 1.02369 4.92893 4.92893C1.02369 8.83418 1.02369 15.1658 4.92893 19.0711Z" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+<path d="M7.40381 16.5967C4.8654 14.0583 4.8654 9.94271 7.40381 7.4043M16.5962 7.4043C19.1346 9.94271 19.1346 14.0583 16.5962 16.5967" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+</svg>