Fix M15: replace role=button divs with native button elements in LibraryView

Both interactive headers in LibraryView used role="button" on divs, which is
invalid when they contain child interactive elements (group header had nested
All/None and Download buttons).

- Group header (groupHead): split into an outer flex div + a native <button>
  (groupToggle style) for the toggle/expand action + sibling Fluent Buttons
  for All/None and Download. The toggle button gets aria-expanded and a
  composite aria-label (title + item count). e.stopPropagation() removed
  from the sibling buttons since they are no longer inside the toggle.

- Source card header (cardHead): converted from a role="button" div to a
  native <button> with width:100%, border:none, and background:transparent.
  aria-label set to source.title. tabIndex/onKeyDown removed (native button
  handles keyboard by default).

typecheck + 242 tests green.

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

src/renderer/src/components/LibraryView.tsx

@@ -124,6 +124,11 @@ const useStyles = makeStyles({
     alignItems: 'center',
     gap: '12px',
     padding: '12px 14px',
+    width: '100%',
+    border: 'none',
+    backgroundColor: 'transparent',
+    fontFamily: tokens.fontFamilyBase,
+    textAlign: 'left',
     cursor: 'pointer',
     ':hover': { backgroundColor: tokens.colorNeutralBackground1Hover }
   },
@@ -164,9 +169,22 @@ const useStyles = makeStyles({
     display: 'flex',
     alignItems: 'center',
     gap: '8px',
-    padding: '6px 4px',
+    padding: '2px 4px'
+  },
+  groupToggle: {
+    display: 'flex',
+    alignItems: 'center',
+    gap: '8px',
+    flexGrow: 1,
+    minWidth: 0,
+    padding: '4px 4px',
+    border: 'none',
+    backgroundColor: 'transparent',
     color: tokens.colorNeutralForeground2,
     cursor: 'pointer',
+    fontFamily: tokens.fontFamilyBase,
+    fontSize: tokens.fontSizeBase300,
+    textAlign: 'left',
     ...shorthands.borderRadius(tokens.borderRadiusMedium),
     ':hover': { backgroundColor: tokens.colorNeutralBackground1Hover }
   },
@@ -413,25 +431,23 @@ export function LibraryView(): React.JSX.Element {
       const groupActionable = groupActionableItems.length
       const allOn = groupActionable > 0 && groupActionableItems.every((it) => selected.has(it.id))
       return (
-        <div
-          className={mergeClasses(styles.groupHead, focus.focusRing)}
-          onClick={() => toggleGroupExpand(row.title)}
-          role="button"
-          tabIndex={0}
-          onKeyDown={(e) => (e.key === 'Enter' || e.key === ' ') && toggleGroupExpand(row.title)}
-          aria-expanded={open}
-        >
-          {open ? <ChevronDownRegular /> : <ChevronRightRegular />}
-          <AppsListRegular />
-          <span className={styles.groupTitle}>{row.title}</span>
-          <Caption1 className={styles.srcSub}>{row.items.length}</Caption1>
+        <div className={styles.groupHead}>
+          <button
+            type="button"
+            className={mergeClasses(styles.groupToggle, focus.focusRing)}
+            onClick={() => toggleGroupExpand(row.title)}
+            aria-expanded={open}
+            aria-label={`${row.title}, ${row.items.length} items`}
+          >
+            {open ? <ChevronDownRegular /> : <ChevronRightRegular />}
+            <AppsListRegular />
+            <span className={styles.groupTitle}>{row.title}</span>
+            <Caption1 className={styles.srcSub}>{row.items.length}</Caption1>
+          </button>
           <Button
             size="small"
             appearance="subtle"
-            onClick={(e) => {
-              e.stopPropagation()
-              toggleGroup(row.items, !allOn)
-            }}
+            onClick={() => toggleGroup(row.items, !allOn)}
           >
             {allOn ? 'None' : 'All'}
           </Button>
@@ -440,10 +456,7 @@ export function LibraryView(): React.JSX.Element {
             appearance="subtle"
             icon={<ArrowDownloadRegular />}
             disabled={groupActionable === 0}
-            onClick={(e) => {
-              e.stopPropagation()
-              downloadGroup(row.items)
-            }}
+            onClick={() => downloadGroup(row.items)}
           >
             Download{groupActionable > 0 ? ` ${groupActionable}` : ''}
           </Button>
@@ -729,13 +742,12 @@ function SourceCard({
   const focus = useFocusStyles()
   return (
     <div className={styles.card}>
-      <div
+      <button
+        type="button"
         className={mergeClasses(styles.cardHead, focus.focusRing)}
         onClick={onToggleExpand}
-        role="button"
-        tabIndex={0}
-        onKeyDown={(e) => (e.key === 'Enter' || e.key === ' ') && onToggleExpand()}
         aria-expanded={expanded}
+        aria-label={source.title}
       >
         {expanded ? <ChevronDownRegular /> : <ChevronRightRegular />}
         <div className={styles.srcIcon}>
@@ -755,7 +767,7 @@ function SourceCard({
             {source.channel && source.channel !== source.title ? ` · ${source.channel}` : ''}
           </Caption1>
         </div>
-      </div>
+      </button>
       {expanded && children}
     </div>
   )