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/Arrows/Round Transfer Diagonal.svg

@@ -0,0 +1,5 @@
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+<path d="M6.5 4L13.8784 12V7.5" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M17.3784 19.8779L9.87842 11.9995V16.4995" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M17 3.33782C15.5291 2.48697 13.8214 2 12 2C6.47715 2 2 6.47715 2 12C2 17.5228 6.47715 22 12 22C17.5228 22 22 17.5228 22 12C22 10.1786 21.513 8.47087 20.6622 7" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+</svg>