[FEATURE] Fullscreen TUI: show a scroll-position indicator (scrollbar) while paging with PgUp/PgDn — keyboard-only, no mouse drag

### What

In `/tui fullscreen` there is no scroll-position indicator of any kind. When I page through a long transcript with PgUp/PgDn I cannot tell where I am, how much is above me, or how far a page moved me.

### Ask — deliberately narrow

Show a scrollbar (a thin track + thumb on the right edge) **while keyboard scrolling is happening** — appear on PgUp/PgDn/arrow scrolling, fade out a moment after the last scroll key — so it works as a position indicator. Keyboard-only is enough; mouse grab-and-drag can be a separate, later task. This is the part of the closed request #62929 that I care about, split out so it can be judged on its own.

### Why this and not "add a scrollbar"

The earlier request (#62929, 4 upvoting comments, closed by the stale bot 2026-09-14) bundled position awareness with mouse drag. The open cousins also bundle things: #88372 asks for a *draggable* scrollbar in fullscreen, #72215 reports scrolling itself broken on Windows, #83840 (stale-labeled) wants mouse-speed navigation. A transient, keyboard-driven indicator is the smallest slice that solves the "where am I in this transcript?" problem, and it avoids the mouse-capture questions those issues raise.

### Steps to reproduce

1. `/tui fullscreen` (or `"tui": "fullscreen"` in settings).
2. Run any session long enough that the transcript exceeds one screen.
3. Press PgUp a few times.
4. Observe: the transcript moves, nothing on screen indicates the position or the remaining length.

### Environment

* Claude Code 2.1.272 (also seen on every version since 2.1.15x).
* Linux (NixOS) in tmux via eternal-terminal / mosh; also macOS 26 iTerm2 + tmux. Same behavior with a config-free tmux server (`tmux -f /dev/null`).
* Renderer: fullscreen.

### Related

* #62929 — original request (closed stale 2026-09-14; this is the position-indicator half of it).
* #88372 — open: draggable scrollbar + position indicator in fullscreen (mouse-centric).
* #72215 — open: fullscreen has no scrollbar and keyboard scrolling does nothing (Windows).
* #83840 — open, stale-labeled: scrollbar for faster mouse navigation.
* #70309 / #61171 / #71491 — closed: native terminal scrollbar lost in the alt-screen renderer.

Start at the `/tui fullscreen` entry point and trace the existing PgUp, PgDn, and arrow-key scrolling behavior. Reproduce the issue with a transcript longer than one screen, then verify that a right-edge track and thumb appear during keyboard scrolling and fade shortly after the last scroll key, without requiring mouse interaction.

Written by the indexing model from the issue text.