Preserve authored mail image proportions in the reader

[stephenw310]

Mail images with explicit width and height attributes could expand into tall squares because the responsive height reset used their intrinsic proportions. A tiny spacer stretched across a message could therefore create an oversized blank header.

Use the sender's numeric dimensions as the default image aspect ratio while preserving responsive sizing and sender CSS overrides. Add light and dark reader coverage for thin headers, spacers, resizing, and explicit CSS ratios. Update the rendering specification.

Validation:

• npm run verify passed: 1,667 unit tests and 394 E2E tests, including type checks, lint, and the production build.
• The regression test fails against the original implementation.
• Inspected reader screenshots in light and dark themes.

The reported email's HTML was unavailable. The fixture reproduces the sizing defect; the exact cause in that message remains unconfirmed.