fix(onboarding): advance install guide after panel opens

[fly1d]

Summary

• advance the install page heading and guidance after the side panel or sidebar opens
• preserve the browser-specific next step across locale refreshes
• announce the updated guidance to assistive technologies

Testing

• node test/run.js (1608 passed)
• npm run test:toolbar-guard (33 passed)
• npm run test:security (60 passed)

Fixes #2732

[vercel]

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[esokullu]

thanks! so what does it show after the sidepanel is opened?

[fly1d]

After the side panel opens successfully, the install page advances to the pinning step:

• The heading changes from “WebBrain installed” to “Pin WebBrain to your toolbar”.
• Chromium shows: “After the side panel opens, use the pin in its header to add WebBrain to your browser toolbar.”
• Firefox shows its browser-specific instructions for pinning WebBrain through the Extensions menu.

The side panel itself opens the normal WebBrain UI; this PR changes the guidance shown on the install page.