fix(mobile): lock viewport, disable iOS bounce, fix 100vh reflow

Root cause: html/body had no height/overflow constraint, so the
webview allowed rubber-band overscroll. .app used min-height: 100vh
which on iOS resolves to the larger viewport (behind URL bar), causing
layout to reflow when the bar collapses.

- index.css: html/body locked to 100% height, overflow hidden,
  overscroll-behavior none; #root set to exact 100dvh
- App.css: .app height: 100dvh (was min-height: 100vh) with
  overflow-y: auto + overscroll-behavior: contain so the .app
  becomes the scroll container
- capacitor.config.json: ios.bounces false, contentInset never

Co-Authored-By: Claude <noreply@anthropic.com>

capacitor.config.json

@@ -6,7 +6,8 @@
     "androidScheme": "https"
   },
   "ios": {
-    "contentInset": "always"
+    "contentInset": "never",
+    "bounces": false
   },
   "android": {
     "allowMixedContent": false