Fix 8 frontend bugs from deep code review

[ppastabtw]

Summary

• Stuck SCANNING state: Wrapped async setTimeout in transitionFromSkeletonsToResults with try/catch so render errors fall back to IDLE instead of leaving the popup stuck with skeleton loaders
• ICS DTEND:null: Fall back to startDate + 1 hour in ICS generator when toDate() returns null for unparseable endTime, preventing invalid .ics files
• AM/PM time display: Added normalizeTimeStr() in dateTime.js to convert AM/PM strings to 24h format before ISO parsing, fixing raw-string fallback display
• Settings button dead after failed click: Removed { once: true } so the listener persists if chrome.tabs.create() fails silently
• End-before-start Google Calendar URL: Added guard in resolveEndDate to clamp to startDate + 1 hour when the resolved end date is before start
• ICS multi-byte fold breakage: Rewrote foldLine to use TextEncoder for byte-accurate folding per RFC 5545 (75 octets, not 75 characters)
• Theme initialization race: Removed the self-executing IIFE from theme.js — both popup.js and settings.js already handle theme loading explicitly
• Deferred poll cleanup: Added deferredTimeoutId tracking in scanPolling.js so clearAll() can cancel deferred polls on popup close

Test plan

• All 188 unit tests pass (npm test)
• Verify popup scan flow: scan page → results load → no stuck skeleton state
• Verify settings button works on repeated clicks
• Download .ics file for event with edge-case times (e.g., unparseable endTime)
• Check AM/PM time strings display correctly in event cards
• Test theme switching in popup and settings page (no flicker)
• Verify events with non-ASCII titles (CJK, emoji) produce valid .ics downloads

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