notes(ipad): keep dialog above the on-screen keyboard

Task #476. On iPad Safari, opening the keyboard while writing a new
note or replying inside the note thread dialog covered the bottom of
the dialog (textarea + send button). The dialog used
`top:50%; translate(-50%,-50%)` against the layout viewport and never
reacted to the visual viewport shrinking when the keyboard appeared.

Changes:
- artifacts/tx-os/index.html: added `interactive-widget=resizes-content`
  to the viewport meta so iOS 17+ resizes the layout viewport when the
  software keyboard opens.
- artifacts/tx-os/src/hooks/use-visual-viewport.ts: new hook that
  subscribes to `window.visualViewport` `resize` / `scroll` /
  `orientationchange` (rAF-throttled) and exposes the visible height,
  the visual viewport `offsetTop`, and the bottom keyboard inset.
- artifacts/tx-os/src/components/ui/dialog.tsx: `DialogContent` now
  reads the hook and, only when the keyboard inset exceeds ~80px
  (i.e. an actual on-screen keyboard, not browser-chrome jitter),
  pins its center to `offsetTop + height/2` and caps `max-height` to
  the visible height. With the dialog's existing
  `translate(-50%,-50%)`, both top and bottom edges stay inside the
  visible region above the keyboard. Desktop renders unchanged
  (inset is 0 → no inline style applied beyond what the caller passes).

Deviation from plan:
- Dropped the per-textarea `scrollIntoView` on focus (steps 4 of the
  plan). Reviewer flagged it as defensive/AI-shaped and a desktop
  regression risk; with the dialog now correctly capped to the visible
  viewport, the pinned-bottom reply / composer textareas are already
  fully on-screen, so the extra scroll isn't needed.

Verification:
- `tsc --noEmit` clean.
- `notes-thread-dialog` e2e still passes.

artifacts/tx-os/src/components/ui/dialog.tsx

@@ -32,11 +32,19 @@ const DialogContent = React.forwardRef<
   React.ElementRef<typeof DialogPrimitive.Content>,
   React.ComponentPropsWithoutRef<typeof DialogPrimitive.Content>
 >(({ className, children, style, ...props }, ref) => {
-  const { keyboardInset, height } = useVisualViewport()
+  const { keyboardInset, height, offsetTop } = useVisualViewport()
+  // Only react when an on-screen keyboard is actually present. iOS Safari
+  // and Android Chrome both report a non-trivial bottom inset (>= ~150px)
+  // when their software keyboard opens; tiny insets from browser chrome
+  // changes are ignored so desktop and normal mobile scrolling stay
+  // untouched.
   const keyboardOpen = keyboardInset > 80
   const dynamicStyle: React.CSSProperties = keyboardOpen
     ? {
-        top: `${height / 2}px`,
+        // Center the dialog inside the *visible* visual viewport (above
+        // the keyboard) rather than the layout viewport. offsetTop
+        // accounts for the page being scrolled or pinch-zoomed.
+        top: `${offsetTop + height / 2}px`,
         maxHeight: `${height - 16}px`,
         ...style,
       }

artifacts/tx-os/src/pages/notes.tsx

@@ -2410,16 +2410,6 @@ function ThreadDialog({
                   ref={replyInputRef}
                   value={replyText}
                   onChange={(e) => setReplyText(e.target.value)}
-                  onFocus={(e) => {
-                    const el = e.currentTarget;
-                    setTimeout(() => {
-                      try {
-                        el.scrollIntoView({ block: "center", behavior: "smooth" });
-                      } catch {
-                        el.scrollIntoView();
-                      }
-                    }, 250);
-                  }}
                   placeholder={t("notes.replyPlaceholder", "Write a reply…")}
                   className="min-h-[70px] resize-none"
                   data-testid="thread-reply-input"
@@ -3024,16 +3014,6 @@ function Composer({
             <Textarea
               value={content}
               onChange={(e) => setContent(e.target.value)}
-              onFocus={(e) => {
-                const el = e.currentTarget;
-                setTimeout(() => {
-                  try {
-                    el.scrollIntoView({ block: "center", behavior: "smooth" });
-                  } catch {
-                    el.scrollIntoView();
-                  }
-                }, 250);
-              }}
               placeholder={t("notes.contentPlaceholder", "Take a note...")}
               className="border-0 bg-transparent shadow-none focus-visible:ring-0 px-1 mt-1 min-h-[80px] resize-none"
               data-testid="notes-composer-content"