feat: make tappable diff lines act as buttons for VoiceOver

Diff lines that accept a review comment used an onTapGesture, which
VoiceOver neither announces as actionable nor reliably activates.
Commentable lines are now wrapped in a Button (.plain style, so they
look identical), giving them the button trait, an "Add a comment on
this line" hint, and proper activation. Non-commentable lines stay plain.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Forji/Forji/Views/DiffView.swift

@@ -60,8 +60,10 @@ struct DiffView: View {
         }
     }
 
+    @ViewBuilder
     private func diffLineRow(line: DiffLine, filePath: String) -> some View {
-        HStack(spacing: 0) {
+        let isCommentable = onLineTap != nil && line.type != .header && line.diffPosition != nil
+        let row = HStack(spacing: 0) {
             HStack(spacing: 2) {
                 Text(line.oldLineNumber.map { "\($0)" } ?? "")
                     .font(.system(.caption2, design: .monospaced))
@@ -88,7 +90,7 @@ struct DiffView: View {
 
             Spacer(minLength: 0)
 
-            if onLineTap != nil, line.type != .header, line.diffPosition != nil {
+            if isCommentable {
                 Image(systemName: "plus.bubble")
                     .font(.caption2)
                     .foregroundStyle(.tertiary)
@@ -100,10 +102,18 @@ struct DiffView: View {
         .padding(.vertical, 1)
         .background(line.type.backgroundColor)
         .contentShape(Rectangle())
-        .onTapGesture {
-            if line.type != .header, line.diffPosition != nil {
+
+        // A tappable line is a Button so VoiceOver announces it and can activate it.
+        if isCommentable {
+            Button {
                 onLineTap?(line, filePath)
+            } label: {
+                row
             }
+            .buttonStyle(.plain)
+            .accessibilityHint("Add a comment on this line")
+        } else {
+            row
         }
     }