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.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "oikos",
-  "version": "0.20.3",
+  "version": "0.20.4",
   "description": "Self-hosted family planner - calendar, tasks, shopping, meal planning, budget and more. Private, open-source, no subscription.",
   "main": "server/index.js",
   "type": "module",