feat: Allow Overriding Timestamp Format in Settings

[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, 12-hour, and locale timestamp format options to app settings

• Adds a new timestampFormat setting ('locale' | '12-hour' | '24-hour') to appSettings.ts, defaulting to 'locale'.
• Introduces timestampFormat.ts with formatTimestamp and formatShortTimestamp utilities that build cached Intl.DateTimeFormat instances per format and seconds-inclusion.
• Threads timestampFormat from ChatView down to MessagesTimeline and PlanSidebar, and applies it in DiffPanel, replacing the previously scattered local formatting logic.
• Exposes a 'Timestamp format' selector with a 'Restore default' button in the Settings UI at _chat.settings.tsx.
• Behavioral Change: formatTimestamp is removed from session-logic.ts; all callers must now use the new module.

^{Macroscope summarized f17bf3f.}

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[juliusmarminge]

looks like rebase went wrong here?

[huxcrux]

Yes you caught me in the middle of a cleanup, it should look beter now :)

[juliusmarminge]

kinda feels like this is appearance related so could go in the same card?

[huxcrux]

kinda feels like this is appearance related so could go in the same card?

That fair, let me push a fix

[huxcrux]

Settings after moving timestamp into appearance:

[juliusmarminge]

as a form of future proofing i'd probably go for a string based format selector. boolean flags are kinda shit and tends to not scale 😆

[huxcrux]

as a form of future proofing i'd probably go for a string based format selector. boolean flags are kinda shit and tends to not scale 😆

Changed the setting to be string based with a dropdown, new look:

[huxcrux]

I made it even better by defaulting to locale (System default) and allowing you to override it.