feat: Support multi-select for combining tab groups

[AmberAlston]

Summary

Enhances the "Combine Groups" feature to support selecting multiple source groups to merge into a single target group.

• Enable multi-select on source group dropdown (Cmd/Ctrl+click for multiple selections)
• Update label to indicate multi-select capability
• All selected source groups merge into target in a single operation
• Target dropdown automatically excludes all selected source groups
• Success message shows count of merged groups

Changes

Modified Files

• src/popup/popup.html - Added multiple attribute to source select, updated label text
• src/popup/popup.js - Handle array of selected source IDs, updated target filtering logic
• src/background/combineGroups.js - Accept array of source group IDs, collect tabs from all sources
• src/background/background.js - Pass sourceGroupIds array to combineGroups
• src/background/combineGroups.test.js - Added tests for multiple source groups
• src/popup/popup.test.js - Updated mocks for multi-select behavior

Test Plan

• Create 3+ tab groups manually in Chrome
• Click extension → "Combine Groups"
• Cmd/Ctrl+click to select multiple source groups
• Verify target dropdown excludes all selected sources
• Click "Combine" and verify all source groups merge into target
• Verify success message shows correct count: "Merged 3 group(s) (15 tabs) into..."
• Verify all source groups are dissolved
• Test single-select still works (backward compatible)

Screenshots