fix(web): handle missing analytics data on every page

[mxriverlynn]

Every data page in skillwalker-web returned a plain-text 500 when the analytics data directory was missing, empty, or only partly populated. The client then showed SyntaxError: Unexpected token 'I', "Internal S"... is not valid JSON instead of its empty-state message.

Why it broke

• The data queries read parquet files with DuckDB without checking they exist. DuckDB throws IO Error: No files found that match the pattern ....
• The SCIL/ACIL routes guarded for No such file or directory, which DuckDB never produces. The route tests passed only because they mocked that invented message. The test-runs and analytics routes had no guard.
• With no onError handler, Hono sent plain text. The pages called res.json() without checking res.ok.

Changes

• Data layer (packages/data): the queries check which parquet files exist before reading them.
  • List queries return [].
  • Detail queries throw the existing "run not found" error.
  • A missing test-results or iteration file gives empty sub-results. Runs without evaluation results now appear with no pass/fail result instead of breaking the page.
• Routes:
  • Removed the error-text matching.
  • InvalidRunIdError now returns 404.
  • A new jsonErrorHandler on app.onError returns { error } JSON for anything unexpected.
• Client:
  • A new fetchJson helper checks the status and falls back to the status text for bodies that aren't JSON. All seven pages use it.
  • The Analytics page now has an empty state.
• Docs: updated docs/web.md error handling and added a missing-parquet section to docs/data.md.

Known limits (documented in docs/data.md)

• A partial write that leaves test-run.parquet without test-config.parquet reads as "no data", not as an error.
• The first write of each parquet file isn't atomic. An interrupted first write can leave a corrupt file that still returns a (now JSON) 500. This predates this PR.

Testing

• New integration tests run every query against a missing data directory, an empty one, and each partial file set. All 20 of them fail on main and pass here.
• New unit tests cover fetchJson, jsonErrorHandler, and the invalid-run-ID 404s.
• make test: 1057 passing.
• Ran the built skillwalker-web against a data directory that doesn't exist. All list endpoints return 200 with empty JSON, and the detail endpoints return a JSON 404, both for unknown and for malformed run IDs.

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