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/Design, Tools/Align Vertical Spacing.svg

@@ -0,0 +1,5 @@
+<svg width="24" height="24" viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path opacity="0.5" d="M22 3L2 3" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+<path opacity="0.5" d="M22 21L2 21" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+<path d="M20 12C20 10.1144 20 9.17157 19.4142 8.58579C18.8284 8 17.8856 8 16 8L8 8C6.11438 8 5.17157 8 4.58579 8.58579C4 9.17157 4 10.1144 4 12C4 13.8856 4 14.8284 4.58579 15.4142C5.17157 16 6.11438 16 8 16H16C17.8856 16 18.8284 16 19.4142 15.4142C20 14.8284 20 13.8856 20 12Z" stroke="black" stroke-width="1.5"/>
+</svg>