feat: merge local datasets in the browser

[Samuel-Hills]

Summary

Adds a local-only workflow for merging two or more existing local LeRobot
datasets from LeLab's Dataset browser.

The source datasets remain unchanged. The merged result is written as a new
local/<name> dataset only after the merge completes successfully.

Included

• Dataset-browser merge dialog for local datasets
• Validation for source selection and output names
• POST /datasets/merge endpoint
• LeRobot merge_datasets integration
• Staged output and cleanup on failure
• Backend and frontend regression tests

Validation

• uv run pytest tests/test_datasets.py -q — 36 passed
• uv run ruff check ... — passed
• npm test — 7 passed
• Focused frontend ESLint — passed
• npm run build — passed

Closes #109.