feat: Add 24-hour timestamp support

[huxcrux]

What Changed

Adds a 24-hour timestamp setting and applies it across chat, diffs, and plan timestamps. It also centralizes timestamp formatting and makes the setting persist safely across reloads.

Why

Timestamps were previously fixed to 12-hour format. Many users, especially outside the US, expect 24-hour time by default or strongly prefer it. This adds a simple preference and applies it consistently across the app without duplicating formatting logic.

UI Changes

Before: screenshot showing 12-hour timestamps in the chat UI

After: screenshot showing 24-hour timestamps in the same UI

Settings: screenshot showing the new Use 24-hour timestamps toggle in Settings

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

Note

Add 24-hour timestamp toggle to app settings and all timestamp displays

• Adds a use24HourTimestamps boolean to AppSettingsSchema (default false) and a toggle in the Settings page under a new 'Time' section.
• Introduces a timestampFormat.ts module with memoized formatTimestamp and formatShortTimestamp helpers that accept a use24Hour flag.
• Propagates the setting through ChatView, MessagesTimeline, PlanSidebar, and DiffPanel so all message and plan timestamps respect the user's preference.
• Fixes parsePersistedSettings to merge defaults with persisted values, so newer settings keys are preserved when loading older stored payloads.

^{Macroscope summarized 713c11a.}

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