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/Weather/Sun.svg

@@ -0,0 +1,11 @@
+<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="6" stroke="black" stroke-width="1.5"/>
+<path d="M12 2V3" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+<path d="M12 21V22" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+<path d="M22 12L21 12" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+<path d="M3 12L2 12" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+<path d="M19.0708 4.9292L18.678 5.32204" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+<path d="M5.32178 18.6782L4.92894 19.0711" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+<path d="M19.0708 19.0708L18.678 18.678" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+<path d="M5.32178 5.32178L4.92894 4.92894" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+</svg>