feat(reader): relocate back-to-location, add recenter control, wrap bottom bar on small windows

- Back-to-location moves from the topbar (where it sat between Back
  and the title, too subtle and disconnected from navigation) into
  both bottom-bar rows, beside the page-back arrow — the natural
  'go back' cluster. New icon: a location pin, clearly distinct from
  the back arrow and page controls. Appears only when the stack has
  a return target; Alt+← unchanged.
- New recenter button in the fixed-layout row (crosshair icon, next
  to zoom): resets pan offsets while keeping the current zoom —
  backed by foliate's new recenter() (1c812e8), which zeroes the
  wrapper translate and re-syncs the spread side.
- Both bottom-bar rows wrap gracefully on narrow windows instead of
  overflowing/h-scrolling: controls are grouped (paging+back | slider |
  fit+zoom+magnifier+recenter | pointer mode | spread | progress+TOC)
  so groups flow to a second line at small widths; the slider shrinks
  first (grow + min-width), everything else stays whole. Fixed-layout
  row drops its overflow-x-auto.

web/src/reader/reader.ts

@@ -1417,6 +1417,12 @@ document.addEventListener("alpine:init", () => {
       this.interactionMode = mode;
       saveSettings({ pdf_interaction_mode: mode });
     },
+    // Reset pan offsets while keeping the current zoom level.
+    recenterView() {
+      if (!this.isFixedLayout || !this.renderer) return;
+      this.renderer.recenter?.();
+      this.renderer.dragOffset = { x: 0, y: 0 };
+    },
     applyFitMode(mode: string) {
       if (!this.isFixedLayout || !this.renderer) return;
       if (mode === "fit-width" || mode === "fit-page") {