feat(analytics): tag cloud transcriptions and report feature usage events

[gabrielste1n]

Desktop side of the workspace usage analytics feature (plan: admin-console-usage-analytics-page.md). Works with the openwhispr-api companion PR; fully backward-compatible with the currently-deployed API (new fields are additive and ignored by the old parser).

What's new

• Feature tagging on cloud requests: /api/transcribe multipart gains feature (dictation/upload), audioDurationSeconds (inline requests), and per-chunk clientEventId (clientTranscriptionId:chunkIndex, retry-safe); /api/streaming-usage gains feature/clientEventId. Uploads deliberately do not transmit clientTranscriptionId (would flip deployed-server chunked uploads to last-chunk-wins quota accounting); the id base is passed out-of-band for event derivation only.
• Meeting usage reporting (new — meetings were previously invisible to analytics in every mode): a per-session snapshot is minted at record start and reported once at stop with word count (seed-excluded for resumed recordings), wall-clock duration, and mode (cloud/byok/local). Cleared on every abort/failure path.
• UsageReportingService (renderer, SyncService conventions): localStorage queue with cross-window Web-Lock serialization, batch flush (≤50) on enqueue/startup/interval, idempotent event ids. Gating: signed-in + managed workspace required for everything; local/BYOK events additionally require the admin's collectLocalUsage setting (from /api/stt-config, honored at enqueue and re-checked at flush so an admin turning collection off drops queued events instead of sending them). Offline/gate-fetch failures queue rather than drop; nothing is ever sent for free individual accounts.
• Privacy disclosure (member-facing): static row in Settings → Privacy when workspace reporting is active — "Your workspace admin can see your word counts and feature usage — never transcript text or audio." Localized in all 10 languages; the existing anonymous-diagnostics toggle is untouched.
• x-openwhispr-version header now sent on authenticated cloud calls (shared header builder) so the console's per-member coverage badges work.
• Mode attribution fix: BYOK→local fallback transcriptions report as local.

Verification

npm run quality-check (eslint, prettier, tsc), check-i18n, and the test suite (194 pass / 0 fail) green. Wire-compat of every new field verified against the deployed API parser.