fix(offline): confirm a dropped connection before going offline

[henry40408]

Problem

Clicking a sidebar link sometimes flipped the connection lamp to Offline. A single failed fetch (e.g. a swap cut off during navigation, via performSwap) or a browser offline event immediately set data-offline.

Fix

• offline.js: going offline now requires a retrying probe (2 attempts, 300ms apart, 5s timeout) against /api/offline/manifest to fail too. Any HTTP response counts as reachable.
• sync()'s failure path, the offline event and networkFailed all use the same confirmation.
• app.js: awaits networkFailed.
• e2e: the "every control ... disabled" step now waits for data-offline, since detection is no longer instantaneous.

Notes

• The flake was not reproduced locally, so this is a fix by cause analysis.
• A real disconnect is reported ~300ms later than before.
• offline_reading.feature: 13/13 scenarios pass.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.18%. Comparing base (cda5833) to head (12f41f9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #581   +/-   ##
=======================================
  Coverage   94.18%   94.18%           
=======================================
  Files          94       94           
  Lines       26856    26856           
=======================================
  Hits        25293    25293           
  Misses       1563     1563           

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