feat: Chrome tab groups, pointer-event drag-and-drop, and duplicate detection

[vivekjain17]

Summary

• Chrome tab groups support — renders native Chrome tab groups as dedicated cards in the dashboard, defaulting to the Tab groups view when named groups are present
• Reliable drag-and-drop — replaces HTML5 DnD (unreliable in extension new-tab pages) with the Pointer Events API; supports dragging group cards, domain cards, and individual tab chips onto group cards to merge tabs into a group
• Duplicate tab detection — highlights duplicate tabs within a group with an amber left-stripe and badge; shows a count in the card header and a "Close N duplicates" CTA button

What changed

File	Change
manifest.json	Added tabGroups permission
app.js	Tab group rendering, pointer-event DnD, duplicate detection logic
style.css	Styles for group cards, drag clone, duplicate chip highlight
index.html	Tab groups / Domains view toggle; "Tab groups" shown first when groups exist

Test plan

• Load extension and open several tabs in different Chrome tab groups
• Verify the Tab groups view shows group cards with correct tab chips
• Drag a group card onto another group card — all tabs should merge
• Drag an individual tab chip onto a group card — tab should join the group
• Open duplicate tabs within a group — amber highlight and "Close duplicates" button appear
• Click "Close N duplicates" — extra tabs close, card updates
• With no named groups, extension defaults to Domains view as before

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