docs(README): Contributing-from-source prereqs list missing Ninja + ripgrep

[aregmii]

Summary

The "Contributing from source" section in README.md lists a "short path" of tools to install before building (Git, Node.js 24+, pnpm 10.10.0, Rust 1.93.0 (rustfmt + clippy), CMake, …). That list is now stale relative to CONTRIBUTING.md's expanded prerequisites table — it's missing Ninja and ripgrep, both of which a fresh contributor will hit during their first build / first git push.

Problem / Context

• Ninja was added to CONTRIBUTING.md's prerequisites in docs(CONTRIBUTING): add missing macOS prereqs (Ninja, x86_64 target, … #1783 (required on macOS to build the bundled CEF helper — without it cef-dll-sys's build script aborts). The README still doesn't mention it.
• ripgrep is being added to CONTRIBUTING.md in fix(lint): make commands-tokens script fail with a clear error when ripgrep is missing + add to CONTRIBUTING prereqs #1867 (required by the lint:commands-tokens pre-push step in app/package.json). The README doesn't mention it either.

A contributor who reads only the README's "Contributing from source" summary (which is intentionally short and points at CONTRIBUTING.md for the long version) will still install the wrong set of tools and hit the same two failures we filed PRs #1783 / #1867 to document.

Acceptance criteria

• README.md "Contributing from source" bullet 1 includes Ninja and ripgrep alongside the existing tool list.

Out of scope

• Adding macOS-specific platform notes (x86_64 target, codesign script) inline in the README — those are correctly delegated to CONTRIBUTING.md via the existing "platform desktop build prerequisites" phrase and the link at the top of the section.
• Re-organizing the README's broader structure — this is a one-line list fix.