fix(dedup): request buffered detector responses and surface scan progress

[bmdy5]

What & why

The duplicate-detection scan asks the LLM for a structured JSON result (candidate duplicate groups), but the request was sent through the streaming chat path. On providers that return an empty body or cut the stream when the model only produces a final JSON payload, the scan hangs with no visible feedback and eventually times out without candidates.

This PR makes the structured detector request non-streaming (buffered) so a complete response always arrives, and adds phase progress reporting so long scans no longer look frozen:

• runDuplicateDetection now accepts an onProgress callback and reports 3 phases: loading (reading pages) / embedding (per-page) / scanning (per batch).
• Batch-loop failures now include the failing batch number in the error.
• Maintenance UI renders a live progress bar + percentage during embedding/scanning phases.

Changes

File	Change
src/lib/dedup-runner.ts	buffered detector request; progress types & callbacks; batch error context
src/lib/dedup_embedding.ts	per-page embedding progress callback (re-added missing signal in CandidateOptions)
src/components/settings/sections/maintenance-section.tsx	live progress bar for scan phases
src/i18n/{en,zh,it,ru}.json	new progress strings (fixed stray double-comma in it/ru)
src/lib/dedup-runner.test.ts	test asserting detector request uses streamingEnabled: false

Verification

• npm run typecheck — passes
• vitest run on dedup-related suites — 88 tests pass

Risks / non-goals

• Only the structured detector call is switched to buffered; streaming chat UX is untouched.
• Progress is best-effort UI feedback; no behavioral change to merge logic.
• Does not change dedup thresholds or grouping logic.

Review focus

• The buffered-request change in dedup-runner.ts (buildDedupLlmCall(..., streamingEnabled: false)) — confirm no streaming-dependent code downstream.
• Progress plumbing is intentionally minimal.

Rollback

Revert this commit; behavior returns to the previous streaming detector path.