Add first-install extension pinning guide

[esokullu]

Summary

• open a focused pinning guide only after a genuine first install
• tailor toolbar guidance for Chrome, Edge, Brave, Vivaldi, Opera, Firefox, and unknown Chromium derivatives
• mirror the responsive guide across Chrome and Firefox, including dark/light themes and reduced-motion support
• localize the new onboarding copy across all 16 supported languages
• keep Firefox from advertising the Chromium-only keyboard shortcut

Why

Newly installed extensions can be hidden in the browser's extension menu, making WebBrain difficult to rediscover. Extensions cannot pin themselves, so the first-install experience now gives users a clear visual rehearsal and browser-appropriate instructions.

The install reason is checked explicitly, so normal extension updates do not reopen the guide.

Validation

• npm test
• 882/882 repository tests passed
• 60/60 injection and security checks passed
• Chrome/Firefox install asset and translation parity checks passed
• git diff --check passed

[esokullu]

@codex pls review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Swish!

Reviewed commit: e649b20489

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