feat(ui): polish popup and tracker menu UX

[kWAYTV]

Summary

• Add popup onboarding copy, loading skeleton, larger tracker switch rows, focus-visible states, and save-error feedback.
• Improve tab semantics and external-link labels in the popup.
• Improve Steam tracker menu empty states, dropdown accessibility, and single-tracker fallback behavior.
• Let the popup subtitle wrap so the full description remains visible.

Walkthrough

trackeroo_popup_ux_quick_wins_final.mp4

Testing

• bun run compile
• bun run check
• bun run check:locales
• bun run test
• bun run build
• Manual Chrome extension walkthrough: loaded unpacked extension, verified subtitle/count, row-click toggling, visible focus ring, and Settings tab.

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