feat: progress elapsed time and remaining ETA with resume continuity

[phaethix]

Summary

• Persist cumulative active generation time in state.json (active_elapsed_seconds) so checkpoint resume continues elapsed instead of resetting to 0
• Expose elapsed_seconds / remaining_seconds on /api/job (linear ETA from progress; hide until ≥5%)
• Show 已用 … · 预计还剩 … on the Web progress UI with a 1s local tick calibrated by poll

Test plan

• .venv/bin/pytest -q green
• Fresh generate: elapsed ticks from ~0; remaining appears after enough progress
• Pause/resume same project_id: elapsed continues (idle gap not counted)
• Supervisor 24h pause still uses session wall-clock only

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