Translate from preserved source, retry transient errors, parallel requests (v0.3.0)

[LockhartKZ]

What & why

Four fixes from a full-codebase review, plus the v0.3.0 bump so the next tag rebuilds the portable builds for all platforms:

1. Preserve the source text — new Cue.source, set at parse time and never mutated. The engine now translates from it instead of cue.text, so the per-line ↻ retranslate and a Cancel-then-Translate re-run always start from the original text instead of re-translating a previous translation (previously the only copy of the source lived in a DOM cell).
2. Retry transient errors — network blips, timeouts, 429s and 5xx now retry up to 2 extra times with a growing delay (isTransientLlmError in client.ts) instead of killing a long run. Auth/4xx errors still fail immediately. Also: cancelling mid-request (LlmError kind aborted) now shows the "Cancelled — partial result kept" toast instead of an error toast.
3. Fix quadratic UI refresh — progress ticks fire once per translated line and each refreshed the whole preview table (full DOM scan + readability regexes per cue → ~4M operations on a 2,000-line file). Now coalesced to at most one refresh per animation frame.
4. Parallel requests option — new engine concurrency param and a persisted "Parallel requests" setting (default 1 = exactly the old sequential behaviour). Higher values run that many batches in flight via a small worker pool: a large speedup on cloud endpoints. On failure the pool stops picking up new batches, in-flight batches finish (partial results kept), and the first error is rethrown.

Trade-off (documented in the UI hint): with parallel requests > 1, context continuity becomes best-effort — a batch sees whatever earlier lines have finished. 1 remains the right setting for local servers.

Releasing: the Windows portable exe in portable/ was rebuilt locally at 0.3.0. macOS/Linux portables can't be built from Windows — after merging, push the v0.3.0 tag and the release workflow builds all three platforms into a draft release.

Engine invariants (check the ones this PR touches)

• Parse → serialize of an unchanged file is still byte-identical
• The LLM still never sees timestamps or container formatting
• Tag/⟦n⟧ placeholder masking is preserved (id-tag + token-set validation intact)
• RTL text remains in logical order (never reversed); output stays UTF-8

Testing

• npm run typecheck passes
• npm test passes — 115 tests (4 new: translate-from-source, transient retry, no-retry-on-401, parallel overlap + correctness)
• UI verified in the vite preview (new field renders, no console errors); full Windows build (npm run tauri build) completed at 0.3.0

Notes

concurrency: 1 takes the identical code path as before (worker pool degenerates to a sequential loop), so default behaviour is unchanged for existing users; saved settings without parallelRequests merge to 1.

🤖 Generated with Claude Code