feat: enable VitePress local search

[dobby-coder]

Summary

• Enables the built-in VitePress local search provider in docs/.vitepress/config.mts.
• Adds a search box to the navbar that indexes all pages at build time. No external service required.

Why

Closes #73. The docs site had no search; users had to rely on the sidebar or browser find-in-page.

Verification

• npm run docs:build succeeds (~6s).
• Built assets include VPLocalSearchBox.*.js and @localSearchIndexroot.*.js, confirming the index is generated.

Reviewer quickstart

git fetch origin && git checkout feat/add-local-search && npm install && npm run docs:dev

Then open the docs and click the search icon (or press /) in the navbar.

[dobby-coder]

Trimmed the PR body: removed the narrative "## Why" paragraph since this is a 3-line config flag flip. Per the agent's no-justification-paragraphs-for-simple-changes rule, small self-evident changes don't need a rationale section beyond the issue link. Kept Summary, Closes #73, Verification, and Reviewer quickstart.

[dobby-coder]

Self-authored PR — can't self-approve, but verified locally: npm run docs:build succeeds in ~6s and the dist output contains VPLocalSearchBox.*.js and @localSearchIndexroot.*.js chunks, confirming the local index is generated. Diff is a minimal 3-line config flag flip with no runtime/security concerns and no external service. Ready for a human approval and merge.