docs: document protoc and mcp-server build steps for source builds

[bsassoli]

Problem

Building from source on a clean machine fails twice before it succeeds, and neither cause is in the README.

1. Missing protoc

lancedb pulls in lance-encoding, whose build script runs prost-build:

error: failed to run custom build command for `lance-encoding v4.0.0`
Error: Custom { kind: NotFound, error: "Could not find `protoc`. ..." }

CI already installs it on all three platforms (.github/workflows/ci.yml, build.yml), so the dependency is known — it just isn't documented for local builds.

2. mcp-server/dist not built

tauri.conf.json bundles ../mcp-server/dist as a resource, so a fresh clone panics:

thread 'main' panicked at build.rs:5:35:
failed to run tauri build script: resource path `../mcp-server/dist` doesn't exist

npm run build:desktop handles this, but the documented flow (npm install → npm run tauri dev) doesn't.

Change

Adds protoc (with the per-platform install command) to the prerequisites line and an explicit npm --prefix mcp-server ci && npm run mcp:build step, in all four READMEs. Docs only — no code changes.