feat: workflow browser with folders, tabs, and card grid

[Lorchie]

What

Reworks how saved workflows are browsed and opened, plus two small UX fixes bundled in.

Workflow browser

• Folders with colors and bookmarks in the Open popup (persisted in localStorage), drag & drop workflows into folders
• Browser-style tabs for open workflows: drag to reorder, middle-click to close, Ctrl+T / Ctrl+W / Ctrl(+Shift)+Tab shortcuts; closing an empty untitled workflow deletes it
• Card grid instead of a flat list in the Open popup: each card shows an SVG mini preview of the graph (nodes tinted by role — inputs green, extensions violet, outputs blue; While containers dashed), search, bookmarks section
• Folder color shown as an ambient halo behind the card preview and a dot on the tab, which pulses while that workflow is running

Fixes

• Title bar maximize/restore icon now follows the real window state (button, title-bar double-click, OS snap gestures)
• nodrag on node input fields — selecting text or opening a select no longer drags the node

Testing

• npm run test:node — 40/40 pass
• npm run lint + tsc --noEmit clean
• Manual: folders/bookmarks/search in the popup, tab reorder and shortcuts, card previews on real workflows, maximize sync on Windows