GLA-40: Extension scaffold

[paoloanzn]

Summary

• add MV3 manifest.json with required permissions (cookies, activeTab) and DevTools entrypoint
• add devtools.html that registers a custom DevTools panel
• add empty panel.html shell for the panel content

Validation

• manifest.json parses as valid JSON
• scaffold is ready to load as an unpacked extension in Chrome for DevTools panel verification

[linear]

GLA-40 Extension scaffold

Set up the bare minimum for the extension to load in Chrome with an empty DevTools panel.

• manifest.json with required permissions (cookies, devtools, activeTab)
• devtools.html that registers the custom panel
• Empty panel page (HTML shell, no functionality yet)

Done when the panel tab appears in Chrome DevTools without errors.