fix: resolve iOS PWA bottom nav positioning via flex layout

Switch .nav-bottom from position: fixed to a flex child of .app-shell.
With position: fixed and will-change: transform (used for the hide/show
animation), iOS compositor layers can misplace the element. As a flex
child (position: relative; flex-shrink: 0) at the end of a height: 100dvh
container the nav is guaranteed to sit flush at the physical screen bottom.

Remove padding-bottom clearance from .app-content, .tasks-page and
.dashboard - no longer needed since the nav no longer overlaps the
content area.

public/styles/dashboard.css

@@ -16,7 +16,7 @@
  * -------------------------------------------------------- */
 .dashboard {
   padding: var(--space-4);
-  padding-bottom: calc(var(--nav-bottom-height) + var(--safe-area-inset-bottom) + var(--space-16));
+  padding-bottom: var(--space-16);
   max-width: var(--content-max-width);
   margin: 0 auto;
 }