feat: chapter chevrons, sticky app frame, and focus mode (0.21.0)

Reader gets chapter-aware chevrons + a persistent app frame +
distraction-free focus mode.

- Single-mode chevrons (and ArrowLeft/Right + j/k) advance pages
  within the chapter and fall through to the adjacent chapter at the
  boundaries. Last page of last chapter / first page of first
  chapter disables the chevron and silent-no-ops on the keypress.
- Continuous-mode gets a fixed bottom bar with prev/next chapter
  buttons; arrows + j/k jump chapters directly.
- `?page=N` and `?page=last` URL query lets the prev-chapter jump
  land on the previous chapter's last page.
- Layout header is fixed at the top; reader nav is sticky just
  below it; both stay visible while scrolling so reading settings
  are always reachable.
- New "Focus" toggle in the reader nav hides the layout header,
  reader nav, and bottom chapter bar with smooth 220ms slide
  animations. Exit via Esc or a small floating Minimize2 button at
  the top-right (low resting opacity, full on hover). Reset on
  reader unmount so it doesn't leak to other pages.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/lib/styles/tokens.css

@@ -60,6 +60,13 @@
     --icon-md: 18px;
     --icon-lg: 22px;
 
+    /* App-frame heights (fixed-position bars at the top and bottom of
+       the viewport). Used by the layout + reader to reserve content
+       space and animate fullscreen mode. Recomputed once if the
+       header padding/font-size ever changes — keep in sync. */
+    --app-header-h: 60px;
+    --reader-bar-h: 56px;
+
     --z-dropdown: 10;
     --z-sticky: 50;
     --z-modal: 100;