Task #634: fix SendNoteDialog centering on iPad PWA with keyboard open

User-reported regression from #630: opening "Send note" on iPad PWA
showed the recipient list pushed to the left side of the screen,
narrower than designed, with the dialog title + search input clipped
off the top. Dismissing the keyboard restored normal centered layout.

Root cause: the `keyboardFallback` branch (iPad PWA, where
visualViewport doesn't update) set `top: "6vh"` in *layout viewport*
coordinates. On iPad PWA Safari, opening the keyboard scrolls the
page down behind the keyboard, so layout-viewport `6vh` ends up
above the visible visual viewport — clipping the top half of the
dialog. Separately, the inline `transform` replaced Tailwind's
translate-x/y on Radix's content, and without an explicit `left:50%`
the animation/RTL combination shifted the box left.

Fix (artifacts/tx-os/src/components/notes/send-note-dialog.tsx):
- `keyboardFallback`: `top: calc(${vv.offsetTop || 0}px + 6vh)` so the
  anchor follows the visible visual viewport when Safari scrolls the
  page behind the keyboard. Add explicit `left: 50%`. Add
  `overflowY: auto` so if dialog content exceeds 50dvh, the dialog
  scrolls internally instead of clipping the header.
- `keyboardActive` branch: also add explicit `left: 50%` defensively
  for the same animation/RTL reason (no behavior change on devices
  where it was already working, but eliminates the asymmetry).

No changes to the shared `dialog.tsx`, composer, or executive-meetings
dialog — scope strictly limited to the regression site.

TypeScript: clean. Pre-existing errors in api-server/src/routes/push.ts
unrelated to this diff.

artifacts/tx-os/src/components/notes/send-note-dialog.tsx

@@ -162,14 +162,31 @@ export function SendNoteDialog({
   const dialogStyle: React.CSSProperties | undefined = keyboardActive
     ? {
         top: vv.offsetTop + vv.height / 2 - KEYBOARD_PREDICTION_INSET,
+        left: "50%",
         transform: "translate(-50%, -50%)",
         transition: "top 120ms ease",
       }
     : keyboardFallback
       ? {
-          top: "6vh",
+          // #634: bind `top` to `visualViewport.offsetTop` so when iPad
+          // PWA Safari scrolls the layout viewport down behind the
+          // soft keyboard, our top anchor follows the visible area
+          // instead of landing above it (which previously hid the
+          // dialog title + search input).
+          top: `calc(${vv.offsetTop || 0}px + 6vh)`,
+          // #634: pin `left: 50%` explicitly. Radix already sets it
+          // via Tailwind, but the inline `transform` below replaces
+          // Tailwind's `translate-x-[-50%] translate-y-[-50%]` so we
+          // also restate the horizontal anchor to defend against any
+          // animation/RTL interplay that was shifting the box left.
+          left: "50%",
           transform: "translate(-50%, 0)",
           maxHeight: "50dvh",
+          // #634: allow the dialog itself to scroll when its content
+          // is taller than the cap — otherwise the header could be
+          // clipped while only the bottom of the list/buttons stayed
+          // visible.
+          overflowY: "auto",
           transition: "top 120ms ease, transform 120ms ease",
         }
       : undefined;