Add on-device app log for failed saves, images, and summaries

[jonocodes]

Why

Failures were only visible in the JavaScript console plus a transient snackbar. On mobile there's no console, so a failed save, a broken image/thumbnail download, or a failed summary left no trace.

What

Adds an app-level event log persisted in IndexedDB and surfaced on the Diagnostics screen.

• Storage: new logs Dexie table (schema v5) and src/utils/logging.ts (recordLog, clearLogs, formatLogsForCopy). Bounded to the last 500 entries; logging is fire-and-forget and never throws.
• Instrumented failures:
  • Article fetch/download and processing failures, including the no-slug case where a per-article fetch.log can't exist (lib/src/ingestion.ts)
  • Image/thumbnail download failures during ingestion (lib/src/ingestion.ts)
  • Summarization failures, both during ingest and manual re-summarize (lib/src/ingestion.ts, ArticleScreen.tsx)
  • Thumbnail loads (tools.ts) and PDF/image/paste ingest errors (SubmitScreen.tsx)
• UI: new "App Log" panel on /diagnostics (Preferences → Diagnostics) with level/category filters and a copy-to-clipboard button so the log can be pasted off a phone. Local only — nothing is uploaded or synced.

Notes

The existing per-article saves/<slug>/fetch.log is untouched and still shown in the article's Edit Info drawer; this is a separate, app-wide log.

Testing

• npx tsc --noEmit — clean
• npm run lint — no new warnings
• npx jest — 281 passed

Docs updated: README FAQ entry and CHANGELOG.

[jonocodes]

Follow-up (6cf4f5f): testing the 429 from the CORS proxy surfaced a gap. A cross-origin response without CORS headers makes fetch reject with a bare TypeError and hides the HTTP status from JS (confirmed: PerformanceResourceTiming.responseStatus is 0 on CORS failure).

Failed downloads now log:

• requestUrl — the resolved URL including the CORS proxy, so you can open it directly and see the status
• status — when the browser exposes it
• proxyUsed / proxyReachable — when the status is hidden, a no-cors probe distinguishes 'proxy answered but browser blocked the response' from 'proxy unreachable'
• hint — points at the likely cause (e.g. 429 rate limiting / missing CORS headers)

Image-download failures also carry requestUrl/status now.