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/Essentional, UI/Slider Vertical Minimalistic.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="M6 8C6 5.17157 6 3.75736 6.87868 2.87868C7.75736 2 9.17157 2 12 2C14.8284 2 16.2426 2 17.1213 2.87868C18 3.75736 18 5.17157 18 8V16C18 18.8284 18 20.2426 17.1213 21.1213C16.2426 22 14.8284 22 12 22C9.17157 22 7.75736 22 6.87868 21.1213C6 20.2426 6 18.8284 6 16V8Z" stroke="black" stroke-width="1.5"/>
+<path d="M21 4.5L21 19.5" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+<path d="M3 4.5L3 19.5" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+</svg>