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/admin/admin-shell.tsx

@@ -37,6 +37,7 @@ import {
 } from "lucide-react";
 
 import { cn } from "@/lib/utils";
+import { SUB_SHELL_GRID_FLOOR } from "@/components/layout/shell-metrics";
 import { useAuth } from "@/hooks/use-auth";
 import { useTranslations } from "@/lib/i18n/context";
 import { isAdminSectionSlug, type AdminSectionSlug } from "./section-slugs";
@@ -304,7 +305,12 @@ export function AdminShell({ active, children }: AdminShellProps) {
           col 1, cards in row 2 / col 2. The nav's first item starts the same
           grid row as the first card, so they align by construction across
           locales and however the heading wraps — no fixed-height spacer. */}
-      <div className="grid gap-6 md:grid-cols-[220px_1fr] md:grid-rows-[auto_1fr]">
+      <div
+        className={cn(
+          "grid gap-6 md:grid-cols-[220px_1fr] md:grid-rows-[auto_1fr]",
+          SUB_SHELL_GRID_FLOOR,
+        )}
+      >
         {/* Heading — desktop only (mobile renders it above the strip). */}
         <div className="hidden md:col-start-2 md:row-start-1 md:block">
           {headingBlock}
@@ -366,14 +372,14 @@ export function AdminShell({ active, children }: AdminShellProps) {
           </ul>
         </aside>
 
-        {/* Main column (cards) — row 2 / col 2. Same
-            `min-h-[calc(100dvh-12rem)]` reserve as `<SettingsShell>`
-            so navigating between admin sub-pages (e.g. system-status
-            → login-overview) does not jump the page height while the
-            new section is still fetching. */}
-        <main className="min-h-[calc(100dvh-12rem)] min-w-0 md:col-start-2 md:row-start-2">
-          {children}
-        </main>
+        {/* Main column (cards) — row 2 / col 2, the grid's `1fr` row, so it
+            stretches to fill the shared `SUB_SHELL_GRID_FLOOR` on the grid
+            (matches `<SettingsShell>`) without over-shooting. The loading-jump
+            floor lives on the GRID, not here: a column-level
+            `min-h-[calc(100dvh-12rem)]` reserve (the pre-#154 shape this used
+            to carry) over-reserves on short sections and on mobile, scrolling
+            the page into a dark band below the last card. */}
+        <main className="min-w-0 md:col-start-2 md:row-start-2">{children}</main>
       </div>
     </div>
   );

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)]";

src/components/settings/settings-shell.tsx

@@ -41,6 +41,7 @@ import {
 } from "lucide-react";
 
 import { cn } from "@/lib/utils";
+import { SUB_SHELL_GRID_FLOOR } from "@/components/layout/shell-metrics";
 import { useAuth } from "@/hooks/use-auth";
 import { useMounted } from "@/hooks/use-mounted";
 import { useTranslations } from "@/lib/i18n/context";
@@ -506,7 +507,12 @@ export function SettingsShell({
           holds across locales and however the title or subtitle wraps. No
           fixed-height spacer to guess. The `gap-6` between rows reproduces
           the `space-y-6` the heading-to-first-card gap used to carry. */}
-      <div className="grid gap-6 md:min-h-[calc(100dvh-10.5rem)] md:grid-cols-[220px_1fr] md:grid-rows-[auto_1fr]">
+      <div
+        className={cn(
+          "grid gap-6 md:grid-cols-[220px_1fr] md:grid-rows-[auto_1fr]",
+          SUB_SHELL_GRID_FLOOR,
+        )}
+      >
         {/* Heading — desktop only here (mobile renders it above the strip). */}
         {headingBlock ? (
           <div className="hidden md:col-start-2 md:row-start-1 md:block">
@@ -570,19 +576,18 @@ export function SettingsShell({
             still prevents the short-loading-state → long-list height jump
             the old reserve guarded, just measured against the real budget.
 
-            v1.4.33 F14 — `pb-24 md:pb-0` reserves a 96 px bottom gutter
-            on `<md` so the last form field on
-            `/settings/account` (Geschlecht / Gender) and
-            `/settings/ai` (Aktiver Provider) is not eaten by the
-            floating mobile bottom-nav. Desktop reverts to the parent's
-            own padding because the bottom-nav doesn't render on `md+`.
+            The column carries NO bottom gutter of its own. AuthShell's
+            `<main>` already reserves `pb-[calc(4rem+safe-area)]` on `<md` to
+            clear the floating bottom-nav, and its wrapper reserves `pb-20` for
+            the Coach FAB. The old `pb-24 md:pb-0` (v1.4.33 F14) double-counted
+            that same 64 px nav clearance and stacked ~96 px of dead scroll
+            below the last card on mobile — removed so the shell owns all
+            bottom-space (over-scroll class, #154 lineage).
 
             v1.16.4 — a `<div>`, not `<main>`: the surrounding AuthShell
             already provides the page's single `<main>` landmark and a
             nested second one is an a11y violation. */}
-        <div className="min-w-0 pb-24 md:col-start-2 md:row-start-2 md:pb-0">
-          {children}
-        </div>
+        <div className="min-w-0 md:col-start-2 md:row-start-2">{children}</div>
       </div>
     </div>
   );