[3.0] Theme split (wave 3, part 4) — Give the width-limited wrappers a shared class

[albertlast]

Description

Part 4 of wave 3 of the #7933 split. Stacked on #9371, #9370 and #9369 — the diff of this one is its last commit.

The rule that keeps the page centred named four wrappers in a single selector:

#top_section .inner_wrap, #wrapper, #header, #footer .inner_wrap {
	max-width: 1200px;
	margin: 0 auto;
	width: 90%;
}

Anything else wanting the same treatment had to be added to that list, and the same list is repeated in responsive.css for the narrow override. It becomes a .content-wrapper class instead, and the width becomes --wrapper-width. This is what the rest of the header work hangs off: once the wrappers are collapsed, the elements that need the width limit are not the ones named above.

Still no visual change. This is the second of the two structural steps, and the reparenting itself comes after it.

Three templates, not one

Themes/default/MaintenanceTemplate.php and the XSLT in Sources/Tasks/ExportProfileData.php each build their own markup but load this same index.css, so their #header, #wrapper and .inner_wrap elements were being centred by that selector too. They get the class as well; without it the upgrade and maintenance pages and every exported profile would lose the width limit and run the full width of the window.

The maintenance page's own #footer has no .inner_wrap and so was never centred by this rule. That is left as it is.

Verification

Board index captured on the parent branch and again here, at two viewport widths so that both sides of the 720px breakpoint are exercised — the wide one is where max-width: 1200px bites and the narrow one is where responsive.css overrides it to width: 100%. The whole DOM is walked, recording tag, getBoundingClientRect and 25 computed properties per element, with max-width among them since that is the property that would break if the class failed to apply:

index  (innerWidth 1401): .content-wrapper 0 -> 4    116 elements, 0 style/geometry differences
narrow (innerWidth  701): .content-wrapper 0 -> 4    116 elements, 0 style/geometry differences

compared 6960 values
style/geometry differences: 0

composer lint cannot run against the two files with classes in this environment — the custom SMF/section_comments fixer calls array_first(), which needs PHP 8.5, and the container is on 8.4. php -l passes on all three files and CI covers the rest.

Issues References (Fixes|Related|Closes)

Related to #7933.