Add dark mode, accessibility, file grouping, and toast notifications

[deactivejb]

Summary

• Dark mode with CSS custom properties, prefers-color-scheme detection, and manual light/dark/system toggle button in the popup
• Accessibility improvements: ARIA roles (role="switch", role="status", role="list"), aria-label on all interactive elements, aria-live region for screen reader announcements, focus-visible outlines for keyboard navigation
• Per-file comment grouping in the popup showing severity badge breakdown per file, with click-to-scroll navigation that highlights the target file
• Toast notifications on the GitHub page when filter settings change
• Severity count badges next to each toggle in the popup
• Quick action buttons: Show All, Hide All, Issues Only

Changes

• 5 files modified: content.js, content.css, popup.html, popup.js, manifest.json
• +737 lines of new functionality

Test plan

• Load extension and verify popup renders correctly in light mode
• Click the moon icon to cycle through system -> dark -> light themes
• Verify dark mode colors apply correctly to all popup elements
• Tab through all interactive elements and verify focus outlines appear
• Use a screen reader and verify ARIA labels are announced correctly
• Open a GitHub PR with CodeRabbit comments and verify severity counts appear
• Verify per-file grouping shows correct files with severity mini-badges
• Click a file in the grouping list and verify the page scrolls to that file
• Toggle a severity filter and verify a toast notification appears on the page
• Click Show All / Hide All / Issues Only and verify correct behavior

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