fix(shell): make AuthShell the single owner of scroll bottom-space

SettingsShell's content column carried pb-24 md:pb-0 that double-counted
the bottom-nav clearance AuthShell's <main> already reserves, stacking
~96 px of dead scroll below the last card on mobile. AdminShell still
carried the pre-#154 min-h-[calc(100dvh-12rem)] column reserve on <main>,
over-reserving on short sections at every breakpoint.

Delete the redundant SettingsShell gutter, move AdminShell's floor off
<main> onto its grid, and factor the budget-derived grid floor into one
shared SUB_SHELL_GRID_FLOOR constant so the two shells can never drift.
Extend the guard to /admin/* and measure against each element's
content-box bottom so a redundant sub-shell pb-* now fails instead of
inflating the allowed budget.

src/components/layout/shell-metrics.ts

@@ -0,0 +1,29 @@
+/**
+ * Shared scroll-floor for the two-column sub-shells (Settings, Admin).
+ *
+ * Scroll height is shell-owned (over-scroll class, #154 lineage): only
+ * `AuthShell` owns the scroll container's height and bottom padding — the
+ * `<main>` clears the mobile bottom-nav and its wrapper clears the Coach FAB.
+ * A sub-shell must NEVER re-declare a bottom gutter (`pb-*`) or a viewport
+ * reserve (`min-h-[calc(100dvh-…)]`) on its own content column: a nested
+ * reserve stacks on the shell budget and scrolls past the last card.
+ *
+ * The one sanctioned reserve is this loading-jump floor, applied to the
+ * sub-shell GRID (not to a column): it keeps the page height stable while a
+ * freshly-navigated section is still fetching, without over-shooting the
+ * viewport on a short section. The `<main>`/cards column is the grid's `1fr`
+ * row, so it stretches to fill exactly and never over-scrolls.
+ *
+ * Both sub-shells derive the floor from THIS single constant so the two can
+ * never drift — Admin previously carried a stale, all-breakpoint
+ * `min-h-[calc(100dvh-12rem)]` column reserve that this de-duplication retires.
+ *
+ * Budget: top bar 4rem + the AuthShell wrapper's `pt-6` (1.5rem) + `pb-20`
+ * (5rem) = 10.5rem. Retune here — and only here — if those shell paddings
+ * change. (Latent follow-up: this constant assumes zero banners above `<main>`;
+ * a locale/offline/demo banner or an iOS PWA safe-area inset makes it
+ * over-reserve. A fully self-accounting `min-h-full` chain would need the
+ * AuthShell content wrapper to pass a definite height through, which is out of
+ * scope for the shell-only over-scroll fix.)
+ */
+export const SUB_SHELL_GRID_FLOOR = "md:min-h-[calc(100dvh-10.5rem)]";