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>

frontend/src/index.css

@@ -34,15 +34,14 @@
   box-sizing: border-box;
 }
 
-html {
+html, body {
+  height: 100%;
+  overflow: hidden;
   overscroll-behavior: none;
+}
+html {
   overflow-x: clip;
 }
-
-body {
-  overflow-x: hidden;
-}
-
 body {
   font-family: 'Outfit', system-ui, sans-serif;
   color: var(--text-main);
@@ -53,13 +52,10 @@ body {
   background-size: cover;
   background-position: center;
   background-attachment: fixed;
-  min-height: 100svh;
-  min-height: 100dvh;
   -webkit-font-smoothing: antialiased;
   -moz-osx-font-smoothing: grayscale;
 }
-
 #root {
-  min-height: 100svh;
-  min-height: 100dvh;
+  height: 100dvh;
+  overflow: hidden;
 }