fix(terminal): partial-history notice only when there is history to load

The "Showing the most recent 1.0 MB of this session. 4.8 MB more may still
be retained." bar covered the top of the terminal on nearly every tab switch,
and for fullscreen claude panes it could never be acted on: `truncated`
measures the server's byte stream, while Load full history asks tmux, which
holds no scrollback for a pane that lives in the alternate screen.

- The capture reads `#{history_size}` in the cursor query it already makes
  and the terminal route returns it as `paneHistoryLines`.
- A reported 0 hides the notice; a positive count replaces the byte gap in
  the message.
- The notice is lazy: it appears once a wheel/touch gesture reaches the top
  of the browser buffer (after the pull that gesture starts), leaves when
  the user scrolls back to live output or switches tabs, and its dismissal
  sticks per session until reload.
- formatHistoryBytes no longer prints "1024 KB" just under 1 MiB.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/api-reference.md

@@ -457,8 +457,13 @@ client that opens many concurrent waits against one session will still hit the c
 What a session's terminal shows, for a client to replay: `data.terminalBuffer`,
 with `source` (`mux-visible`, `mux-full-history` or `history`), `truncated`,
 `truncationReason`, `fullSize`, and `captureCols`/`captureRows` when the pane's
-geometry was read. The capture runs synchronous tmux calls on the server; the
-`Server-Timing` header reports `capture`, `prepare` and `total`.
+geometry was read. `paneHistoryLines` (present whenever the body is a pane capture)
+is the number of scrollback rows tmux holds above the visible frame, which is the
+most a `full=1` request can add. `truncated` describes the byte stream instead: for a
+pane with `paneHistoryLines: 0` (a fullscreen CLI in the alternate screen) the bytes a
+`tail` cut dropped are earlier repaints that no request returns. The capture runs
+synchronous tmux calls on the server; the `Server-Timing` header reports `capture`,
+`prepare` and `total`.
 
 | Query | Meaning |
 |---|---|