[codex] Merge global content search

[hackjutsu]

Summary

Ports the previously merged global content search work onto current master. PR #602 had been merged into the intermediate codex/built-in-theme-options branch, but that branch was later closed without merging to master, so this brings the feature into the release line.

• Adds exact, case-insensitive search across loaded snippet metadata and downloaded file content.
• Adds a shared search-record builder used by sync, create/edit flows, fixture data, and lazy single-gist fetches.
• Updates the search modal with result counts, match-source labels, matched phrase highlighting, content excerpts, filename chips, and updated timestamps.
• Keeps complete content search behind the shipped snippet.downloadAll config behavior; the old unshipped gist.downloadAll path remains ignored.
• Updates README, docs, wiki, i18n copy, and focused tests.
• Adapts the port to current master by using the existing fetch-based GitHub API stack and local formatRelativeTime utility instead of the stale moment import.

Validation

• npm run test:unit -- tests/utilities/search.test.js tests/utilities/config.test.js tests/utilities/githubApi.test.js
• npm run lint
• npm test
• npm run test:smoke
• git diff --check

Rendering

npm run test:smoke passed, including the search fixture. The local search fixture screenshot showed the updated search overlay placeholder: Search descriptions, tags, file names, and content....