Add session-based timed mode with results screen

[RagibAhashan]

Summary

• Timer is now a session-wide countdown rather than per-challenge: press space to start, runs continuously across every challenge until it expires.
• On expiry the editor is replaced by a results view: accuracy, avg time, avg score, fastest, plus a per-challenge time chart and a restart button.
• Adds avg-time tracking; the elapsed timer now starts when a challenge becomes interactive, so motion-only challenges no longer record 0 ms.

Closes #1.

Test plan

• Toggle timed mode on → "Press space to start" overlay appears with the editor blurred.
• Press space → countdown starts, editor focuses.
• Solve several challenges; countdown ticks continuously across them.
• On expiry: results view shows stats grid + time-per-challenge bars (fastest highlighted).
• Click restart → fresh shuffle, gate re-armed, stats cleared.
• Disable timed mode mid-session → countdown stops; toggle back on → gate reappears.
• Untimed mode unaffected.

[vercel]

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[wailay]

I think the timed mode should remain per challenge not a global timed session like monkeytype, because a challenge usually represent a vim motion. That way, the user can improve their vim motions speed and efficiency.

Also that would be great if you could keep that feature you added in the current behaviour:

by a results view: accuracy, avg time, avg score, fastest, plus a per-challenge time chart and a restart button.