reader pin UX: push-pin icon, toggle, left/right split

Per user feedback on the reader action bar: use the push-pin glyph (not
the chain/link), make the pin button toggle the pinned state (tap to pin
current, tap again to unpin) via ReaderNav.togglePin, and split the
layout so the pin/unpin toggle sits on the left while jump-to-pin joins
the navigator (back/forward) on the right — left toggles, right
navigates. The action button gains an active (accent) state for the
pinned indicator.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -40,9 +40,9 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
   `.clideignore` pair — the single ignore knob clide owns. (T-52)
 - `files.walk` command — a recursive, ignore-pruned, capped flat file listing
   of the workspace, backing quick-open and search. (T-51, T-52)
-- Sidebar readers (markdown + decision) gain a chrome action bar: back/forward
-  history, a single-slot pin (one click sets/replaces; one click to return),
-  and an edit pencil that opens the current doc in the editor. (T-189, T-190, T-191)
+- Sidebar readers (markdown + decision) gain a chrome action bar: a pin/unpin
+  toggle on the left, then a navigator (back, forward, jump-to-pin) and an edit
+  pencil on the right. (T-189, T-190, T-191, T-198)
 
 ### Fixed