fix(extension): pin the popup width to 360px on the root element

[vineethkrishnan]

Problem

The popup opened in a window about twice its usual width. Measured from the report screenshot, the window's content area was 720x539: the height matches the CSS body height (540), while the width is exactly 2x the 360px body, which floated centred in an oversized viewport.

Nothing in the extension asks for that size:

• the only windows.create (extension/entrypoints/background.ts:1688) requests 384x640, and never requested anything wider in its history (400x620 -> 376x600 -> 384x640);
• the shipped 1.27.1 bundle still carries body{width:360px;height:540px;...}, so the fixed box is intact;
• Chrome's stored prefs hold no matching remembered bounds.

So the width was chosen by the browser, not by us. The page gave it room to: the width was pinned only on body, html had a height but no width, and index.html carried <meta name="viewport" content="width=device-width, initial-scale=1.0">, which hands the layout viewport to the device rather than to our CSS.

Change

• Pin width: 360px (and the auto margins that centre it) on html, so the popup's width cannot be inferred from whatever viewport the browser hands the page.
• Drop the width=device-width viewport meta from the popup document; an extension popup has no device viewport to adapt to.
• body keeps its own 360x540 box, and the sub-viewport scroll fallback added in fix(extension): recognize the unlock window as an extension page #347 is untouched.

Verification

CSS has no automated coverage here, so this was checked against a real build. Built the extension and served .output/chrome-mv3/popup.html in a 1920px-wide Chrome viewport:

viewport: [1920, 934]   dpr: 2
html:     360 wide, left 780
body:     360 x 540, left 780

The UI renders as a centred 360px column with the themed background filling the window, instead of stretching or drifting. Built CSS confirms html{width:360px;height:100%;margin:0 auto} and the body theme block is unchanged.