ui : fix attachment text preview clipped when a line is longer than the container (#30806)
In the attachment preview, SyntaxHighlightedCode's wrapper (overflow-auto) was not the element that ended up owning horizontal overflow: the inner code box kept its content clipped at the wrapper's width, so a single line longer than the preview pane was cut off with no scrollbar (scrollWidth == clientWidth while the code's own content measured tens of thousands of px).
Give the pre/code boxes their intrinsic width (width: max-content, min-width: 100%) so long-line overflow propagates to the scrolling wrapper. Verified in the running WebUI: with a 6000-char line, wrapper scrollWidth goes from 990 (clipped, unscrollable) to 50449 with a working horizontal scrollbar; short content still fills the container exactly as before (min-width: 100%).
Fixes #30796