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/Users/User Cross.svg

@@ -0,0 +1,6 @@
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+<circle cx="12" cy="6" r="4" stroke="black" stroke-width="1.5"/>
+<path d="M15 13.3271C14.0736 13.1162 13.0609 13 12 13C7.58172 13 4 15.0147 4 17.5C4 19.9853 4 22 12 22C17.6874 22 19.3315 20.9817 19.8068 19.5" stroke="black" stroke-width="1.5"/>
+<circle cx="18" cy="16" r="4" stroke="black" stroke-width="1.5"/>
+<path d="M16.6665 14.6665L19.3332 17.3332M19.3335 14.6665L16.6668 17.3332" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+</svg>