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/Broken/Design, Tools/Mirror Left.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 d="M3 12C3 16.2426 3 18.364 4.31802 19.682C5.63604 21 7.75736 21 12 21M12 3C7.75736 3 5.63604 3 4.31802 4.31802C3.50241 5.13363 3.19151 6.25685 3.073 8" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+<path d="M11 21H15C17.8284 21 19.2426 21 20.1213 20.1213C21 19.2426 21 17.8284 21 15V9C21 6.17157 21 4.75736 20.1213 3.87868C19.2426 3 17.8284 3 15 3H11" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-dasharray="2.5 3"/>
+<path d="M12 22L12 2" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+</svg>