Switch pane resize handle to pointer events

internal/notesium/web/app/pane.js drove the sidebar/pane resize
via mousedown + window-level mousemove/mouseup, which doesn't
fire on touch (browsers only emulate mouse for taps, not drags).

Replaced with pointer events:
- @pointerdown on the handle (covers mouse, touch, pen).
- setPointerCapture so we keep receiving pointermove/pointerup
  events when the pointer drifts off the handle. This eliminates
  the need for document-level listeners and avoids stuck-drag
  states when the user releases outside the window.
- pointermove + pointerup + pointercancel listeners on the
  captured target only — when the capture ends they're removed
  regardless of whether the user is still on top of the handle.
- Filter on event.pointerId so a second simultaneous touch
  (e.g., a multi-finger gesture) cannot hijack the in-progress
  resize.
- event.button !== 0 guard rejects right-click / middle-click.
- touch-action: none on the handle so the browser doesn't try
  to interpret a horizontal drag as a page scroll.

CodeMirror's internal mousedown handlers in note.js / preview.js
are left alone — those are link-click guards, not drags, and
CodeMirror's own pointer support handles touch internally.

Closes #20.

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

internal/notesium/web/app/pane.js

@@ -4,9 +4,10 @@ var t = `
     <slot></slot>
   </div>
   <div v-show="resizing" v-text="paneWidth + 'px'" class="absolute bottom-0 right-0 p-4 text-gray-400 text-xs"></div>
-  <div @dblclick="setDefaultWidth()" @mousedown="startResize"
+  <div @dblclick="setDefaultWidth()" @pointerdown="startResize"
     :class="direction == 'right' ? 'left-full pl-1.5 mr-1.5' : 'right-full pr-1.5 ml-1.5'"
-    class="z-50 absolute group inset-y-0 cursor-ew-resize flex items-center">
+    class="z-50 absolute group inset-y-0 cursor-ew-resize flex items-center"
+    style="touch-action: none;">
     <div class="h-8 w-1 rounded-full group-hover:bg-gray-300"></div>
   </div>
 </div>
@@ -26,29 +27,49 @@ export default {
       startResizeClientX: null,
       startResizePaneWidth: null,
       resizing: false,
+      activePointerId: null,
     }
   },
   methods: {
     startResize(event) {
+      // Only respond to primary button / single touch / pen.
+      // event.button is 0 for primary mouse and is also 0 for
+      // touch / pen via PointerEvent.
+      if (event.button !== undefined && event.button !== 0) return;
       this.startResizeClientX = event.clientX;
       this.startResizePaneWidth = this.paneWidth;
       this.maxWidth = this.$el.parentElement.offsetWidth - 50;
       this.resizing = true;
+      this.activePointerId = event.pointerId;
       event.preventDefault();
-      document.addEventListener('mousemove', this.doResize);
-      document.addEventListener('mouseup', this.stopResize);
+      // Capture the pointer so we keep getting move/up events even
+      // when the pointer leaves the handle. This replaces the
+      // window-level mousemove/mouseup listeners and handles touch
+      // and pen identically.
+      try { event.target.setPointerCapture(event.pointerId); } catch (_) {}
+      event.target.addEventListener('pointermove', this.doResize);
+      event.target.addEventListener('pointerup', this.stopResize);
+      event.target.addEventListener('pointercancel', this.stopResize);
     },
     doResize(event) {
+      if (event.pointerId !== this.activePointerId) return;
       let draggedDistance = event.clientX - this.startResizeClientX;
       if (this.direction === 'left') draggedDistance = -draggedDistance;
       const newWidth = this.startResizePaneWidth + draggedDistance;
       if (newWidth <= this.maxWidth && newWidth >= this.minWidth ) this.paneWidth = newWidth;
     },
-    stopResize() {
+    stopResize(event) {
+      if (event && event.pointerId !== this.activePointerId) return;
       this.resizing = false;
       this.savePreferredWidth();
-      document.removeEventListener('mousemove', this.doResize);
-      document.removeEventListener('mouseup', this.stopResize);
+      const target = event && event.target;
+      if (target) {
+        try { target.releasePointerCapture(this.activePointerId); } catch (_) {}
+        target.removeEventListener('pointermove', this.doResize);
+        target.removeEventListener('pointerup', this.stopResize);
+        target.removeEventListener('pointercancel', this.stopResize);
+      }
+      this.activePointerId = null;
     },
     setDefaultWidth() {
       this.paneWidth = this.defaultWidth;