[Week 07] Run and Monitor view: runtime status and node summaries

[bobdingAI]

Tracker issue derived from the student's accepted proposal (proposal.pdf).

Week 07 goal (from proposal):

Weeks 6-8: Run and Monitor View — "display runtime status and node summaries."

This issue covers the middle of the Weeks 6-8 block: build the Run and Monitor View showing dataflow lifecycle state (running / stopped / error) and per-node runtime summaries, fed from the live channel established in Week 06.

Definition of done: Run/Monitor view renders dataflow lifecycle state and node-level summaries that update live.

Mentor: @bobdingAI
Student: @DGHX12345
Fork: https://github.com/DGHX12345/gsoc2026-dora-studio

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Refine scope with @bobdingAI in the first weekly meeting. Split into smaller issues if needed.

[DGHX12345]

Summary

• Replace the placeholder README with English setup, demo walkthrough, API surface, validation, and current limitation notes for the prototype.
• Add a configurable frontend API base URL, make Dashboard the default landing page, and wire Export report to download a Markdown prototype snapshot.
• Clean up Run & Monitor summary cards so aggregate CPU and pending counts are derived from active node data instead of hardcoded demo values.

Test Plan

• cargo fmt --manifest-path backend/Cargo.toml --check
• cargo test --manifest-path backend/Cargo.toml
• npm --prefix frontend run build
• Backend/frontend local smoke checks for /api/health, /api/dataflows, and frontend root
• Headless Chrome walkthrough for Dashboard default view, six-page navigation, dark mode, Run & Monitor aggregates, and Export report download