fix(graph-ui): surface indexing-job failures instead of silently completing

[DeusData]

fix(graph-ui): surface indexing-job failures instead of silently completing

Distilled from #549 (thanks @sahil-mangla!).

Refs #524

What does this PR do?

The backend /api/index-status reports status:"error" plus an error message for failed indexing jobs (src/ui/http_server.c, handle_index_status), but the frontend's IndexProgress treated any non-"indexing" state as successful completion. When an indexing job failed (e.g. the indexer subprocess was OOM-killed with exit code 137), the spinner simply vanished: no database, no project in the UI, no error message.

This PR renders a visible error banner instead — failed path + error text + a Dismiss button — while preserving the existing success flow.

Changes beyond the original PR

• Empty-jobs guard restored. The original PR dropped the data.length > 0 condition, so an empty jobs list would have triggered onDone(). The backend keeps finished jobs listed as "done"/"error" (handle_index_status only skips idle slots, and a job's slot is populated before the POST /api/index 202 reply), so an empty list mid-index can only mean transient state loss (e.g. server restart) — it must not be treated as completion. The guard is re-added with a comment and covered by a test.
• i18n. The new user-facing strings are routed through the existing i18n system (projects.indexingFailed, common.dismiss) with EN and zh entries, instead of hardcoded English.
• Tests adapted to backend semantics. The success-flow test now models completion as a "done" entry (matching the backend) instead of an empty list, and assertions reference the i18n messages.

The backend dev-mode asset-hosting change from the original PR is intentionally not included (separate concern; this PR stays scoped to the UI fix).

Testing

• graph-ui vitest suite: 4 new IndexProgress cases (progress rendering, success flow, empty-jobs guard, error banner + dismiss), all green; full suite green.
• Red-first verified: on main's StatsTab.tsx the error-banner test fails; the empty-jobs guard test fails on the unguarded variant of the fix.
• tsc -b clean; no C sources touched.

Checklist

• Every commit is signed off (git commit -s)
• Tests pass locally
• New behavior is covered by tests