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/List/Bill.svg

@@ -0,0 +1,6 @@
+<svg width="24" height="24" viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M2 2H22" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+<path d="M4 2H20V12.27C20 13.6091 20 14.2787 19.7319 14.8691C19.4638 15.4595 18.9595 15.9004 17.951 16.7822L15.951 18.5309C14.0685 20.177 13.1272 21 12 21C10.8728 21 9.93152 20.177 8.04897 18.5309L6.04897 16.7822C5.04046 15.9004 4.5362 15.4595 4.2681 14.8691C4 14.2787 4 13.6091 4 12.27V2Z" stroke="black" stroke-width="1.5"/>
+<path d="M8.5 13L15.5 13" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+<path d="M8.5 8L15.5 8" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+</svg>