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/Bold Duotone/Medicine/Pills 2.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 opacity="0.5" d="M13.4645 20.5355C15.4171 22.4882 18.5829 22.4882 20.5355 20.5355C21.5118 19.5592 22 18.2796 22 17C22 15.7204 21.5118 14.4408 20.5355 13.4645C18.5829 11.5118 15.4171 11.5118 13.4645 13.4645C12.4882 14.4408 12 15.7204 12 17C12 18.2796 12.4882 19.5592 13.4645 20.5355Z" fill="black"/>
+<path opacity="0.5" d="M3 7C3 9.76142 5.23858 12 8 12C9.38071 12 10.6307 11.4404 11.5355 10.5355C12.4404 9.63071 13 8.38071 13 7C13 4.23858 10.7614 2 8 2C6.61929 2 5.36929 2.55964 4.46447 3.46447C3.55964 4.36929 3 5.61929 3 7Z" fill="black"/>
+<path d="M12.0562 17.75H21.9438C21.9813 17.5015 22 17.2508 22 17C22 16.7492 21.9813 16.4985 21.9438 16.25H12.0562C12.0187 16.4985 12 16.7492 12 17C12 17.2508 12.0187 17.5015 12.0562 17.75Z" fill="black"/>
+<path d="M3.97388 4.03454L10.9654 11.0261C11.1676 10.8769 11.3582 10.7128 11.5355 10.5355C11.7128 10.3582 11.8769 10.1676 12.0261 9.96541L5.03454 2.97388C4.83232 3.12307 4.64175 3.28712 4.46444 3.46444C4.28712 3.64175 4.12307 3.83232 3.97388 4.03454Z" fill="black"/>
+</svg>