feat(inbox): show relative timestamp on signal cards

[oliverb123]

Summary

• Adds a reusable RelativeTimestamp component in components/ui/ that renders relative time (e.g. "1 hour ago") with the exact locale datetime in a hover tooltip.
• Renders it in SignalCardHeader so every signal in the inbox report detail view shows its timestamp at the top right of the card, alongside the existing Verified badge.
• Removes the now-redundant duplicate timestamp from the bottom of GenericSignalCard.

Test plan

• Open a report in the signals inbox with mixed signal types (GitHub, Zendesk, error tracking, session problem, generic) and confirm each card shows the relative timestamp top-right.
• Hover the timestamp and confirm the full locale datetime appears in a tooltip.
• Confirm the generic fallback card no longer renders the duplicate timestamp below the body.
• Confirm cards with missing/invalid timestamps don't render an empty slot or "Invalid Date" text.

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Created with PostHog Code

[greptile-apps]

Prompt To Fix All With AI

Fix the following 2 code review issues. Work through them one at a time, proposing concise fixes.

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### Issue 1 of 2
apps/code/src/renderer/components/ui/RelativeTimestamp.tsx:28
Trailing space in className when `className` prop is omitted. The template `"... ${className ?? ""}"` produces a trailing space when no className is provided. This is a minor spacing issue per the project's template string conventions. Using a conditional insertion is cleaner.

```suggestion
        className={`shrink-0 text-(--gray-10) text-[11px]${className ? ` ${className}` : ""}`}
```

### Issue 2 of 2
apps/code/src/renderer/components/ui/RelativeTimestamp.tsx:30
**Stale relative time display**

`formatRelativeTimeLong` is called once at render time and computes `Date.now() - date.getTime()`, but the component has no mechanism to re-run as wall-clock time advances. A user who leaves the inbox view open will see the label freeze (e.g. "1 minute ago" stays forever). Consider driving a periodic re-render with a `useState` + `useEffect`/`setInterval` that ticks every minute, or using a shared time-tick context.

_{Reviews (1): Last reviewed commit: "feat(inbox): show relative timestamp on ..." | Re-trigger Greptile}