feat(ui): cap page columns at 1536px, left-aligned, on one page edge - #2574

🟢 Confidence 10/10 · safe to merge. The gutter resolves to the same 16px the base padding already gave, so only the 1536px cap and the hairline change width or paint.
_{no findings · tests not needed · risk medium}

Moves the page edge to a --page-gutter variable, caps the default column at 1536px left-aligned, and swaps the scrolled header's shadow for a hairline. Geometry matches the base at every breakpoint, so it is safe to merge.

• --page-gutter on the page root pads the top bar, sticky area, rail and scroll area
• dashboardPage defaults to a left-aligned 1536px wide column; reading is removed
• The scrolled sticky area draws border-border-subtle instead of shadow-popup
• Traces list/detail, service map, dashboard view, widget editor and metric detail pass width: "full"

Fixed since the last review

• ✅ F1 · Filter rail keeps px-4, so at lg it sits 8px left of the top bar

_{fd31eb9 · Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}