feat(dashboard): command + response/error preview on timeline hover (#514)

[dolho]

Summary

Timeline bars on the dashboard now surface a preview on hover so it's possible to scan executions without clicking each one. Tooltip is three lines:

• Header: trigger type, status, duration (unchanged)
• Cmd: first 80 chars of the originating message
• Out or Error: first 80 chars of the response on success, or the error text on failure

Pipeline

• Backend (services/task_execution_service.py): on success completion, merge details.response_preview = sanitized_resp[:200] into the chat_start activity row. Failed activities already populate activity.error via the existing complete_activity(error=...) calls.
• Store (stores/network.js): timeline mapper now exposes command_preview (from details.message_preview, already populated at start), response_preview, and the existing error on every event.
• Component (components/ReplayTimeline.vue): threads the new fields through agentActivityMap and bars. getBarTooltip renders the new lines via previewLine() — collapses whitespace, truncates to 80 chars with an ellipsis, drops the line entirely when input is empty so the tooltip stays compact.

Uses the existing SVG <title> primitive (same as the schedule-marker tooltip), so newline rendering is consistent. No new API round-trip — the preview data piggybacks on /api/activities/timeline via the existing details JSON field.

Acceptance criteria

• Hover shows two preview lines (cmd + response/error). Empty payloads silently skip the line so short tasks stay tidy.
• Failed executions show Error: … in place of Out: ….
• Works for chat, scheduled, agent-triggered, MCP, paid, and public executions — all funnel through task_execution_service.execute_task.
• No additional API round-trip — preview ships in the existing timeline payload.
• Long content truncated at 80 chars with …; whitespace collapsed.
• Mobile graceful degradation: SVG <title> is hover-only; tap on a bar still opens the execution detail page (existing behaviour).

Test plan

• Backend write path verified: triggered an execution, queried the DB row → details.response_preview present after my change (synthetic preview rendered for visual smoke, then cleaned up).
• API serves it: GET /api/activities/timeline returns details.response_preview in the activity payload.
• Frontend build passes (vite build clean).
• Visual eyeball required by reviewer: dashboard → Timeline mode → hover an execution bar. Confirm three lines render with \\n newlines.

Notes for reviewer

I exercised the data pipeline end-to-end against the running stack. The visual tooltip rendering itself wasn't browser-tested in this session — the SVG <title> newline pattern is already proven by the schedule-marker tooltip in the same component, and the build is clean, but please give it a 5-second eyeball before merging.

Closes #514.

🤖 Generated with Claude Code

[dolho]

[vybe]

LGTM — data pipeline verified (message_preview at task_execution_service.py:324,349). Please do a quick hover-check on the timeline before calling it done.