fix: stop Data Manager double-scrolling and double-padding inside Layout's main (#4145)

[atomantic]

Summary

/data (Data Manager) appeared in neither EXACT_FULL_WIDTH_PATHS nor FULL_WIDTH_PATH_PREFIXES, so Layout gave it the default overflow-auto p-4 md:p-6 <main>. The page itself is a flex flex-col h-full shell with its own bordered header bar over a flex-1 overflow-auto p-4 body — so the page's scroller nested inside <main>'s scroller, and both applied padding.

Chose option 1 from the issue (register the route as full-width) over option 2 (flatten the page into a normal scrolling page), because a sticky header bar over a scrolling body is exactly the shape the full-width branch exists for — the same shell as /tribe, /rapid-reader, /timeline, and /songbook, all of which are registered full-width.

One correction to the issue's phrasing of option 1: it suggested also dropping the page's h-full/overflow-auto wrapper "since <main> would now handle the scroll container". It would not. The full-width branch renders <main> as a bare relative overflow-hidden, and client/src/CLAUDE.md is explicit that a full-width page must own an internal overflow-y-auto or it clips below the fold. So the page's shell is kept as the single scroll container; what's removed is <main>'s competing one.

Registered as an exact path rather than a prefix: a /data prefix would also match the /datadog redirect route.

Changes:

• client/src/components/Layout.jsx — add /data to EXACT_FULL_WIDTH_PATHS.
• client/src/pages/DataManager.jsx — min-h-0 on the flex column and on the body, shrink-0 on the header bar, so a tall body scrolls rather than stretching the shell past <main>. This also makes the live shell byte-for-byte equivalent to what PageSkeleton reserves.
• client/src/pages/DataManager.jsx — the PageSkeleton call's fullHeight / padded / barClassName="p-4" / bodyClassName="p-4" props are the correct set for the new shell and are unchanged; a comment now records why (its root is flex flex-col min-h-0 h-full, bar shrink-0 … p-4, body flex-1 min-h-0 overflow-y-auto p-4 — matching the loaded page exactly). Dropping them here would have reintroduced the mismatch in the other direction, since the full-width <main> supplies neither scroll nor padding.

Test plan

• client/src/components/Layout.test.jsx — new Layout — Data Manager scroll mode integration case asserting /data renders the overflow-hidden main with no overflow-auto/p-4, while /devtools/datadog stays padded+scrolling; plus ['/data', true], ['/datadog', false], ['/devtools/datadog', false] rows in the isFullWidthRoute classification table.
• client/src/pages/DataManager.test.jsx — new DataManager full-width shell (#4145) suite asserting the loaded page is a non-scrolling h-full column with exactly one direct-child scroll region (flex-1 min-h-0 overflow-auto p-4), that the header bar is shrink-0 and outside it, and that the loading skeleton reserves the same shape.
• Bypass probe: commenting out the /data entry fails both new Layout assertions (2 failures), confirming the tests are not vacuous.
• cd client && npm test — 640 files / 7766 tests passing.
• cd client && npx biome lint --error-on-warnings on the four touched files — clean.

Closes #4145