Add immediate snapshot fetch when polling starts

[Copilot]

Addresses feedback on #2: the UI showed no snapshots until the first 3-second polling interval elapsed when the agent started working.

Changes

• Immediate fetch on polling start: Call fetchSnapshots() when agentState === 'working' to populate snapshots without delay
• Retain final fetch on stop: Keep existing behavior to capture last screenshot when agent finishes

if (agentState === 'working') {
  startPolling(taskId);
  // Fetch immediately when polling starts to show snapshots without delay
  fetchSnapshots();
} else {
  stopPolling();
  // Do a final fetch when the agent finishes to capture the last screenshot
  fetchSnapshots();
}

Polling continues every 3 seconds while agent is working.

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