Extension popup opens with inconsistent initial size (Chrome auto-sizing issue)

### Description
The EduAid browser extension popup opens with inconsistent initial dimensions
across sessions and browsers (Chrome / Edge).

This occurs because extension popups are auto-sized by the browser based on the
document's initial layout. Using responsive or `min-width` / `min-height` based
CSS does not guarantee a stable initial size for extension popups.

As a result:
- The popup sometimes opens smaller than intended
- UI elements appear cramped or clipped on first render
- Layout may shift after content loads

This issue is specific to the **browser extension popup** and is independent of
web app responsiveness fixes (e.g. `eduaid_web` changes).

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### Expected Behavior
The extension popup should always open at a consistent base size
(e.g. 420×600), with internal scrolling for overflow content.

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### Proposed Solution
Hard-code width and height on `html`, `body`, and `#root` for popup pages only,
which is the recommended approach for Chrome/Edge extension popups.

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### Environment
- Browser: Chrome / Microsoft Edge
- Surface: EduAid browser extension popup