#201 fix(demo): recover safely after server disconnect

Summary

• treat the inspector as disconnected until /api/state returns a successful, valid state payload
• disable browser-action controls while disconnected while preserving an existing trace for export
• add an explicit Retry connection control that performs one GET and only re-enables actions after validation
• never replay a failed POST or browser mutation during recovery

Coverage

• initial offline load
• disconnect after live state, with the prior trace retained and exportable
• non-2xx and malformed-JSON retry responses
• successful reconnect with exactly one GET and no replayed POST

Validation

• uv run ruff check .
• uv run pytest — 32 passed
• node --check jev_ultrafast/static/app.js
• uv build
• git diff --check origin/main...HEAD

Prepared from upstream main at 1231850a0bf1a0c0341fe408ef1668dbbfdfac46. Open PRs #183 and #186 touch the demo UI but do not implement disconnected-state gating or GET-only recovery; this draft keeps that behavior independently reviewable.

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Summary by cubic

Makes the browser demo inspector recover safely from server disconnects, so a failed action is never replayed and an existing trace stays exportable.

• The inspector is treated as disconnected until /api/state returns a valid state payload, so it no longer trusts malformed or non-2xx responses.
• Browser-action controls are disabled while disconnected, but the current trace remains visible and exportable.
• Adds a Retry connection button that performs one GET and re-enables actions only after the state payload validates.
• Failed POSTs and browser mutations are never replayed during recovery.