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/Building, Infrastructure/Buildings.svg

@@ -0,0 +1,9 @@
+<svg width="24" height="24" viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M22 22L2 22" stroke="#1C274C" stroke-width="1.5" stroke-linecap="round"/>
+<path d="M21 22V6C21 4.11438 21 3.17157 20.4143 2.58579C19.8285 2 18.8857 2 17 2H15C13.1144 2 12.1716 2 11.5858 2.58579C11.1143 3.05733 11.0223 3.76022 11.0044 5" stroke="#1C274C" stroke-width="1.5"/>
+<path d="M15 22V9C15 7.11438 15 6.17157 14.4142 5.58579C13.8284 5 12.8856 5 11 5H7C5.11438 5 4.17157 5 3.58579 5.58579C3 6.17157 3 7.11438 3 9V22" stroke="#1C274C" stroke-width="1.5"/>
+<path d="M9 22V19" stroke="#1C274C" stroke-width="1.5" stroke-linecap="round"/>
+<path d="M6 8H12" stroke="#1C274C" stroke-width="1.5" stroke-linecap="round"/>
+<path d="M6 11H12" stroke="#1C274C" stroke-width="1.5" stroke-linecap="round"/>
+<path d="M6 14H12" stroke="#1C274C" stroke-width="1.5" stroke-linecap="round"/>
+</svg>