art: harden zoom pointer lifecycle (Codex sign-off)

- Reset dragging on every exit path (enterZoom, fit, Escape, lightbox-close effect) so a
  drag interrupted by Escape/Fit can't carry the grabbing state into the next session.
- Drag ends on pointerup/pointercancel/lostpointercapture (dropped pointerleave, which
  fought the capture) so a drag genuinely continues outside the image.
- dragStart guards e.button===0; track the captured pointerId and release only when
  hasPointerCapture() — no double-release throws.
- a11y: slider aria-valuetext ("150 percent").

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/routes/art/+page.svelte

@@ -36,10 +36,10 @@
   let tx = $state(0), ty = $state(0);   // pan offset in px (translate)
   let dragging = $state(false);
   let zoomBoxEl, zoomImgEl;             // refs for clamping pan to the image bounds
-  let lastX = 0, lastY = 0;
+  let lastX = 0, lastY = 0, activePointer = null;
 
-  function enterZoom() { zoomLevel = 1.5; tx = 0; ty = 0; zoomed = true; }
-  function fit() { zoomed = false; }    // back to the framed gallery view
+  function enterZoom() { zoomLevel = 1.5; tx = 0; ty = 0; dragging = false; zoomed = true; }
+  function fit() { dragging = false; zoomed = false; }   // back to the framed gallery view
   function clampPan() {
     if (!zoomImgEl || !zoomBoxEl) return;
     const maxX = Math.max(0, (zoomImgEl.offsetWidth * zoomLevel - zoomBoxEl.clientWidth) / 2);
@@ -55,7 +55,8 @@
     clampPan();
   }
   function dragStart(e) {
-    dragging = true; lastX = e.clientX; lastY = e.clientY;
+    if (e.button !== 0) return;         // primary button only
+    dragging = true; lastX = e.clientX; lastY = e.clientY; activePointer = e.pointerId;
     e.currentTarget.setPointerCapture?.(e.pointerId);
   }
   function dragMove(e) {
@@ -64,18 +65,22 @@
     lastX = e.clientX; lastY = e.clientY;
     clampPan();
   }
+  // Ends drag on pointerup / pointercancel / lostpointercapture (NOT pointerleave, which
+  // fights the capture). Release only what we actually hold, so no double-release throws.
   function dragEnd(e) {
     dragging = false;
-    e.currentTarget.releasePointerCapture?.(e.pointerId);
+    const el = e.currentTarget;
+    if (activePointer !== null && el?.hasPointerCapture?.(activePointer)) el.releasePointerCapture(activePointer);
+    activePointer = null;
   }
 
   function onKey(e) {
     if (e.key !== 'Escape') return;
-    if (zoomed) zoomed = false;         // Escape steps out of inspection first, then closes
+    if (zoomed) { zoomed = false; dragging = false; }   // Escape steps out of inspection first, then closes
     else if (zoom) zoom = false;
   }
   // Leaving the lightbox always resets the inspector, so re-opening starts framed.
-  $effect(() => { if (!zoom) { zoomed = false; zoomLevel = 1.5; tx = 0; ty = 0; } });
+  $effect(() => { if (!zoom) { zoomed = false; dragging = false; activePointer = null; zoomLevel = 1.5; tx = 0; ty = 0; } });
   // Move focus to the lightbox when it opens, so Escape/Enter work and focus is trapped sanely.
   $effect(() => { if (zoom && lightboxEl) lightboxEl.focus(); });
 
@@ -273,7 +278,8 @@
     {#if zoomed}
       <!-- gallery inspector: DRAG to pan; zoom with the toolbar; Fit/Esc to exit -->
       <div class="lb-zoom" class:dragging bind:this={zoomBoxEl}
-           onpointerdown={dragStart} onpointermove={dragMove} onpointerup={dragEnd} onpointerleave={dragEnd}
+           onpointerdown={dragStart} onpointermove={dragMove} onpointerup={dragEnd}
+           onpointercancel={dragEnd} onlostpointercapture={dragEnd}
            role="img" aria-label="{art.title}, magnified — drag to pan">
         <img bind:this={zoomImgEl} src={art.image_url_large || art.image_url} alt={art.title}
              style="transform:translate({tx}px,{ty}px) scale({zoomLevel})" draggable="false" />
@@ -281,7 +287,8 @@
       <div class="lb-tools" role="group" aria-label="Zoom controls">
         <button class="lb-t" onclick={() => setZoom(zoomLevel - 0.1)} aria-label="Zoom out">−</button>
         <input class="lb-slider" type="range" min="1" max="4" step="0.1" value={zoomLevel}
-             oninput={(e) => setZoom(+e.currentTarget.value)} aria-label="Zoom level" />
+             oninput={(e) => setZoom(+e.currentTarget.value)}
+             aria-label="Zoom level" aria-valuetext="{Math.round(zoomLevel * 100)} percent" />
         <button class="lb-t" onclick={() => setZoom(zoomLevel + 0.1)} aria-label="Zoom in">+</button>
         <span class="lb-pct">{Math.round(zoomLevel * 100)}%</span>
         <button class="lb-fit" onclick={fit}>Fit</button>