fix: prevent event propagation issues in sidebar and editor

- Add type=button to Cancel/Save buttons in editor to prevent form submission
- Add href=# and return false to quick-links to prevent default anchor behavior
- StopPropagation was already present in sidebar handlers (verified)
- Document view buttons already had type=button (verified)

These fixes ensure:
- Library/tag clicks don't bubble up and trigger wrong handlers
- Quick-link buttons don't cause unexpected navigation
- Editor buttons don't accidentally submit forms

public/js/views/editor.js

@@ -41,9 +41,9 @@ export async function renderEditor(app) {
   function render() {
     appEl.innerHTML = `
       <header class="app-header">
-        <button class="btn btn-ghost" onclick="handleCancel()">Cancel</button>
+        <button type="button" class="btn btn-ghost" onclick="handleCancel()">Cancel</button>
         <span style="flex:1;margin-left:16px">${isNew ? 'New Document' : 'Editing: ' + escapeHtml(formData.title)}</span>
-        <button class="btn btn-primary" onclick="handleSave()">Save</button>
+        <button type="button" class="btn btn-primary" onclick="handleSave()">Save</button>
       </header>
       <main class="main-content">
         <div class="editor-container">