fix(util): format collapsibleDate time without locale dependency

[WolfieLeader]

Summary

Fixes #1494

describeCollapsibleDate() called date.toLocaleTimeString() which produces 24-hour format on non-US systems, causing garbled output like "14:30, January 15, 2023" instead of "2:30 PM, January 15, 2023".

Replaced with locale-independent manual formatting using getHours(), getMinutes(), getSeconds():

• hours % 12 || 12 for correct 12-hour conversion (midnight→12, noon→12)
• hours < 12 ? " AM" : " PM" for suffix
• Reuses existing pad() helper
• Removed now-unused timeWithUnnecessarySeconds regex (caught by eslint no-unused-vars)

Test plan

• All 8 existing collapsibleDate tests pass unchanged (year-only, day, minutes, seconds, milliseconds, midnight, noon, AM/PM)
• Verified all edge cases: midnight (h=0→12 AM), noon (h=12→12 PM), hour boundaries
• pnpm prChecks passes (format, lint, typecheck, full test suite)

[pullfrog]

Clean, correct fix. The hours % 12 || 12 idiom handles all edge cases (midnight→12 AM, noon→12 PM), and conditionally building the time string is simpler than the previous approach of calling toLocaleTimeString() then stripping/rewriting parts of its output. The timeWithUnnecessarySeconds regex removal is a natural consequence. No issues found.

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[WolfieLeader]

Awesome pullfrog!