#455 Allow deselecting chosen problem card on click

Summary

Support deselecting an active problem card by clicking it again in the lobby problem picker.

When the currently chosen card is clicked:

• Releases the manual selection state by resetting manualProblem = false and keptDraw = null.
• Invokes recommend() if history is ready to fall back to the default recommendation engine, or setProblem(null) if it is not.
• Preserves the existing searchVisibilityException state when selecting/reselecting.

Verification

Added browser test in tests/browser/lobby.test.js to verify:

1. Clicking a problem card applies manual selection (snap.card === cardId, prompt updated).
2. Clicking the active card releases the manual override (manualProblem = false, keptDraw = null) and re-triggers recommend(), clearing the manual selection prompt.

Closes #392

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Summary by cubic

Clicking the currently chosen problem card in the lobby picker now releases the manual selection and restores the default recommendation.

• Resets manualProblem and keptDraw, then re-triggers recommend() when history is ready, or clears the problem and its recommendation text otherwise.
• Preserves the searchVisibilityException state when reselecting.
• Adds a browser test that toggles the active card off and verifies the picker falls back to the default recommendation.

Closes #392.