Move Chrome extension into its product directory

[patrickschiller]

What changed

• Move the Manifest V3 extension from the repository root to chrome-extension/sourcebraid.
• Update tests, release packaging, contributor guidance, and English/German installation documentation for the new path.
• Keep the unpacked extension self-contained by bundling the PDF-support payload inside its directory, with a regression test that verifies those copies match the canonical repository files.
• Preserve the Chrome Web Store ZIP layout, including manifest.json at the archive root.

Why

The Chrome client now follows the same explicit product-directory layout as ios/ and codex-plugin/sourcebraid/, while remaining loadable without a build step.

Impact

Developers should select chrome-extension/sourcebraid when using Chrome's Load unpacked action. There are no changes to browser permissions, GitHub scopes, network endpoints, storage formats, deletion behavior, or credential handling.

Validation

• node --test tests/capture-utils.test.js (15 passed)
• python3 -m unittest discover -s tests -p "test_*.py" (35 passed)
• python3 scripts/build_chrome_package.py --output /private/tmp/sourcebraid-chrome-layout-test.zip
• Inspected the generated ZIP: 15 allowlisted files, with manifest.json at its root.