fix(vad): guard unknown field limits and normalize context in whisperVadConfig - #1926

Thanks for the contribution, @hsusul! Closing after tracing this end to end: clampVadField has exactly one production call site — sanitizeWhisperVadConfig (src/helpers/whisperVadConfig.js:21) — which iterates Object.keys(DEFAULTS), and the DEFAULTS and LIMITS key sets in src/constants/whisperVad.json are identical, so LIMITS[key] can never be undefined; the renderer-facing whisper-vad-set-config IPC payload is additionally whitelisted in _setWhisperVadSettings (ipcHandlers.js:697) before it can touch this path. resolveContextSileroEnabled likewise has exactly one production call site (ipcHandlers.js:721 via _resolveWhisperVadOptions), whose five callers all pass the hardcoded literals "dictation" (ipcHandlers.js:2880, 7509), "noteRecording" (2693, 5822), and "meeting" (7214) — the context string is never renderer-, file-, or network-supplied, so mixed-case or padded values (and the new "note_recording" alias, which appears nowhere in the codebase) never occur. Per our convention (validate once at real trust boundaries, trust internal callers past that point), these guards would only convert a future caller bug into a silently unclamped passthrough instead of a loud failure. Closing #1925 along with this.