fix(learn,ui): the open assistant field holds still: no flicker, no sliding away, no resizing under the keyboard (ADR 0021, D69)

Inside a studyset the field flickered as it opened and closed: the
floating location switched between a raised centre and the centre,
which Scaffold animates by scaling the button out and in. It keeps one
location now, raised above the licence line by padding. While open it
no longer slides away as the studyset or the Library scrolls
(ScrollHidingFab.pinned, new), and the keyboard covers Home, the
Library, a studyset and a session rather than shrinking them; the field
lifts itself by as much of the keyboard as reaches above it.
NavigationFrame no longer resizes for the keyboard itself, leaving it
to each tab's own scaffold, which still does by default (Todo's,
Train's and Meditate's tests pass unchanged).

Whats-New: Inside a studyset, the question field opens without flickering, stays while you scroll, and the screen no longer jumps when the keyboard comes.
Whats-New-De: In einem Lernset öffnet sich das Fragefeld ohne Flackern, bleibt beim Scrollen stehen, und der Bildschirm springt nicht mehr, wenn die Tastatur kommt.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

docs/adr/0021-private-instance-and-assistant.md

@@ -531,6 +531,15 @@ assistants' effort work:
     Only the button of the screen at the router's location unfolds, so a tab kept alive
     behind another holds no second field. Sending empties it everywhere. How far an
     assistant looks stays each field's own, starting at only what is chosen (D50).
+  - **Open, the field holds still** (the developer's review of 4 Nov 2026, inside a
+    studyset): it unfolds in place, raised above the licence line by padding rather than
+    by another location of the scaffold's, which the scaffold animates by scaling the
+    button out and in (the flicker); it never slides away as the content scrolls
+    (`ScrollHidingFab.pinned`); and the keyboard covers the screen rather than shrinking
+    it (`resizeToAvoidBottomInset` off on Home, the Library, a studyset and studying while
+    it is open), the field lifting itself by as much of the keyboard as reaches above it.
+    halkyon_ui's `NavigationFrame` leaves the keyboard to each tab's own scaffold for
+    this, every app's screens resizing as before by default.
 
 As built on 26 Oct, for v0.22.0 (build 25, which the server needs: `minimumAppBuild` 25,
 as older apps read limits per grant), D57–D63 read so where they left something open: