Make image viewer truly full screen

PLAN.md

@@ -1044,6 +1044,11 @@ dev-updater (Kotlin 2.4.x, CMP 1.11.x, JDK 21).
      different composable in a different part of the tree, so the old subtree
      and its open dialog go. Somebody looking at a screenshot was thrown back
      to the transcript because the session made another tool call.
+   - **The image viewer fits against the whole physical display, including the
+     system-bar regions** (2026-09-12). It hides the status and navigation bars
+     independently when the fitted, zoomed or panned image reaches them, and
+     restores either one when it does not. `100%` recenters at one bitmap pixel
+     per screen pixel; opening still fits the whole image first.
    - **All transcript text is selectable, from one `SelectionContainer`
      around the whole list.** Not per row: a transcript is one body of text,
      so a selection has to run from a reply into the tool output under it —