fix(web): sidebar reste fixe sur page longue, page Liste de courses centrée

- AppLayout: .app-layout passe de min-height:100vh à height:100vh +
  overflow:hidden, et .app-content devient le vrai conteneur de scroll
  (overflow-y:auto). Avant, une page plus haute que le viewport faisait
  défiler tout le document, entraînant la sidebar avec lui au lieu de la
  garder fixe.
- ComingSoonPage (page "Liste de courses"): ajoute margin:0 auto, comme
  .settings-page, au lieu de rester collée au bord gauche avec un grand
  espace vide à droite sur les écrans larges.

apps/web/src/layouts/AppLayout.scss

@@ -7,8 +7,14 @@
 // No `@use` of the theme partial needed here — see HomePage.scss's identical
 // note: every token below is a CSS custom property, available at runtime.
 
+// `height` (not `min-height`): the layout must stay pinned to exactly the
+// viewport, never grow taller than it — otherwise a tall page scrolls the
+// whole document, dragging the sidebar out of view with it. `overflow:
+// hidden` backs that up so nothing can force the document itself to grow;
+// `.app-content` below is the one place that actually scrolls.
 .app-layout {
-  min-height: 100vh;
+  height: 100vh;
+  overflow: hidden;
   display: flex;
   background: var(--color-background);
 }
@@ -27,6 +33,10 @@
   background: var(--color-surface);
   border-right: 1px solid var(--color-border);
   transition: width 0.15s ease;
+  // Own scroll, independent of `.app-content` — only matters on a very
+  // short viewport with a long nav, but keeps the rail from ever forcing
+  // `.app-layout` past 100vh.
+  overflow-y: auto;
 
   &__top {
     display: flex;
@@ -315,6 +325,11 @@
   // Content can scroll independently of the sidebar (e.g. a long planning
   // table) without the fixed-width rail ever needing to shrink.
   min-width: 0;
+  // The actual scroll container: `.app-layout` is pinned to `height: 100vh`
+  // above, so a page taller than the viewport scrolls in here instead of
+  // scrolling the document (which would drag the sidebar along with it).
+  min-height: 0;
+  overflow-y: auto;
   padding: var(--space-xl);
 }
 

apps/web/src/pages/ComingSoonPage.scss

@@ -2,8 +2,13 @@
 // Styles for ComingSoonPage — shared by every stub section page.
 // =============================================================================
 
+// Centered, not pinned to `.app-content`'s left edge — same reasoning as
+// `.settings-page` (settings-pages.scss): on a wide desktop viewport a
+// left-aligned `max-width` here just left a lopsided gap down the right
+// side instead of framing the placeholder copy.
 .coming-soon-page {
   max-width: 40rem;
+  margin: 0 auto;
 
   p {
     color: var(--color-text-muted);