fix(theme): make the header readable on the dark backdrop

[skjnldsv]

Started as a screenshot refresh and turned into a bug fix.

The viewer is dark whatever theme the server runs. Only the backdrop was. The header kept the light theme's --color-main-text, so the filename and the actions rendered at #222 on black:

fg rgb(34,34,34)  bg rgb(0,0,0)  contrast 1.32

WCAG asks 4.5 of normal text. It reads at all in the old screenshot only where the photograph lights it from behind. The dark palette is now scoped to the modal, unless a handler asked for a light backdrop, which takes it to 21.

The theme test is how this lasted: it asserted the backdrop had gone dark and said nothing about what sits on it. It now checks the contrast too, and I reverted the fix to confirm it fails, at exactly 1.32.

The screenshot

Retaken against the fixed header, so it shows the rotate button that arrived in beta.11 rather than the September layout.

The playground's own file list has grown long enough with all the format fixtures that it now shows beneath the modal, so it is hidden for the portrait. That is the harness, not the product.

The picture sitting high

Worth recording what it was, because it was not what it looked like. There was no overlap and no opacity:

header     0 → 44      (--header-height: 44px)
container  44 → 756
img        44 → 756    viewport 800

Equal bands above and below, so it was centred. It read high because the top band was filled by the header while the matching bottom band was empty, and the picture began exactly where the bar ended.

There are now eight pixels under the header, and the band below is left as it was, so the top one carries the header and the gap:

header     0 → 44
img        52 → 756

390 unit, 88 e2e.

👾 This pull request was assisted by Claude Code, commits carry an Assisted-by trailer.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.80%. Comparing base (0dd399b) to head (33a5460).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #60   +/-   ##
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  Coverage   88.80%   88.80%           
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  Files          39       39           
  Lines        2743     2743           
  Branches      564      564           
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  Hits         2436     2436           
  Misses        300      300           
  Partials        7        7           

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