Add comment search, export, and floating summary widget

[jbingham17]

Summary

• Text search within CodeRabbit comments with real-time yellow highlighting
• Export comments to Markdown, JSON, or CSV formats via the popup
• Severity count badges displayed next to each toggle in the popup
• Quick action buttons: Show All, Hide All, Issues Only for fast bulk toggling
• Draggable floating summary widget on GitHub PR pages showing live severity counts and hidden totals
• Widget visibility toggle in the popup to show/hide the floating widget
• Added downloads permission for file export support

Changes

• 5 files modified: content.js, content.css, popup.html, popup.js, manifest.json
• +778 lines of new functionality

Test plan

• Load extension and open a GitHub PR with CodeRabbit comments
• Type in the search box and verify yellow highlights appear in matching comments
• Verify severity counts update correctly next to each toggle
• Click "Show All", "Hide All", "Issues Only" and verify correct filtering
• Click each export button (Markdown, JSON, CSV) and verify downloaded files
• Verify the floating widget appears in bottom-right corner with correct counts
• Drag the widget to a new position and verify it stays
• Close the widget via the X button and verify the popup toggle reflects the change
• Toggle widget visibility from the popup and verify it shows/hides

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