feat: billing CSV export, Nova-3 label fix, tenant ID metering, key validation #4767

Originally written by @ccantynz-alt on GitHub.
Imported from https://github.com/ccantynz-alt/voxlen/pull/34

Summary

• Billing CSV export — lawyers and accountants can now export time/billing records as CSV from the Clients panel. Per-client export (download icon on each card) and all-clients export (Export CSV in header). Uses Tauri file-save dialog with browser download fallback.
• Tenant ID metering — X-Tenant-ID header now sent on all api.voxlen.com proxy calls (STT + grammar) so the backend can meter and bill per account
• Voxlen key validation — onboarding wizard validates the API key on blur against api.voxlen.com/v1/auth/verify, auto-populates voxlenTenantId from the response, shows green/red inline feedback
• preserve_tone Rust fix — was hardcoded to true; now reads from user setting
• shortcut_correct_grammar added to Rust AppSettings so it round-trips through save/load
• Input component success prop — green border + message for positive validation states
• Nova-3 label fix — STT_ENGINES constant updated from Nova-2 to Nova-3

Test plan

• Open Clients panel with entries → "Export CSV" button visible in header
• Click per-client download icon → CSV downloads with correct rows and TOTAL
• Paste Voxlen API key in onboarding → green "Key verified" on blur
• Toggle Preserve Tone off in Settings → verify grammar config respects it
• npm test — 98 tests pass

https://claude.ai/code/session_011MFNHeFKrhfuSrEYVgrdyr

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