fix: prevent mobile page scroll when typing with keyboard open

iOS Safari scrolls the document to bring xterm's hidden textarea into
view when the user types, pushing the entire UI off-screen. Fix with:
- CSS position:fixed on .app when keyboard is visible
- window.scroll listener to reset scroll position as safety net
- scroll reset in onKeyboardShow before and after fit/resize

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/web/public/mobile.css

@@ -428,6 +428,18 @@ html.mobile-init .file-browser-panel {
     position: relative;
   }
 
+  /* When keyboard is visible, pin the app container to the viewport.
+     iOS Safari scrolls the page to bring the focused input (xterm's hidden
+     textarea) into view, even with overflow:hidden. position:fixed prevents
+     the browser from scrolling the document under the app. */
+  .keyboard-visible .app {
+    position: fixed;
+    top: 0;
+    left: 0;
+    right: 0;
+    bottom: 0;
+  }
+
   /* Ultra-compact session tabs — .tabs-two-rows override needed to match
      specificity of .session-tabs.tabs-two-rows in styles.css (0,2,0) */
   .session-tabs,