Fix popup expansion at root width

[LTDigor]

Summary

• Lock the popup root to 360 px instead of relying on a scrollbar gutter.
• Prevent Chrome from expanding the popup viewport around a still-360 px body.
• Replace the misleading width test with the actual root-width invariant.
• Add a rendered Chrome regression that opens the real popup UI at an 800 px host viewport, adds the five rules from the reported screenshots, expands the list, and verifies root/body width remains 360 px.

Root cause

The body was fixed at 360 px, but the root element remained auto-sized. When Chrome widened the extension-popup viewport to 800 px after the list became taller than the popup limit, the body/cards stayed 360 px and the root filled 800 px, creating the large blank area on the right. scrollbar-gutter: stable did not constrain the root and therefore did not fix the bug.

Validation

• Browser regression demonstrated RED before the fix: root width was 800 px.
• Browser regression is GREEN after the fix: root/body stay 360 px collapsed and expanded.
• Headed extension action popup: outer width 374 → 374 px; root/body 360 → 360 px.
• npm test: 102 tests passed, 0 failed, 0 skipped.
• npm run package: Chromium and Firefox archives built.
• No relevant console warnings or errors.

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