docs(harper.js): add Chrome extension example and docs page

[leepokai]

Note

Per Harper's policy on agent PRs: this PR was authored with an LLM-based agent (Claude Code), human-directed and reviewed. End-to-end verification is described below.

Closes #3628

What

• New example packages/harper.js/examples/chrome-extension/: a minimal Manifest V3 extension whose popup lints a textarea with harper.js and shows the bundled version — the "tiny example… opens a popup… and displays the harper-core version" asked for in Docs: Add an intro for how to use harper.js inside a Chrome extension #3628.
• New docs page /docs/harperjs/chrome-extension covering the three MV3 pitfalls that make this "next level complicated":
  1. Extensions may not load remote code, so the CDN approach doesn't apply — build.js copies the harper.js dist into the extension, and binaryInlined avoids a separate fetch for the wasm binary.
  2. The default extension CSP blocks WebAssembly compilation — the manifest opts in with wasm-unsafe-eval.
  3. WorkerLinter spawns its Web Worker from a blob URL, which the extension CSP blocks — so the example uses LocalLinter.
• Sidebar entry under harper.js after "CDN" in packages/web/vite.config.ts.

Verification

• Built the example against published harper.js@2.7.0 and loaded the resulting dist/ as a real unpacked extension in Chromium (Playwright, --load-extension):
  • popup loads with zero page/console errors, i.e. the wasm engine compiles under the manifest CSP
  • the intentionally flawed sample text yields 3 lints (two "Incorrect indefinite article", one proper-noun hint)
  • the footer renders harper.js 2.7.0
• biome check clean on the new files.

The version question raised in the issue is addressed at the bottom of the docs page: there is no runtime API exposing the engine version, but harper.js and harper-core are versioned together (both currently 2.7.0), so the package version identifies the engine.