fix(electron): clamp render preset length and reject unknown encode casing

[ubugeeei]

Problem

`validateRenderStartPayload` strictly checks the preset against an allowlist (good), but doesn't bound the string length before doing so. A caller can send a megabyte-sized `preset` string; `RENDER_PRESETS.has` short-circuits to `false` but the validator allocates the full buffer first. Similarly, `encode` accepts only exact `"H264" | "H265"` — but "h264" (lowercase) silently fails with a generic message that could be friendlier.

Expected

• Length check (e.g. `<= 32 chars`) before the allowlist check on `preset`.
• Distinct error message when `encode` is a non-canonical string ("h264", "h.264", etc.).

Acceptance

• Validator rejects oversized preset strings early.
• Validator returns a clearer message for casing issues on encode.
• Covered by tests (see chore: add .nvmrc to pin the Node version #60).