fix(FN-679): use merge-base diff in modal changes tab

- Updated /tasks/:id/diff endpoint to use merge-base comparison
- Changed from HEAD~1..HEAD to ...HEAD syntax
- Added fallback to HEAD when merge-base fails
- Verified fix matches /tasks/:id/file-diffs endpoint pattern

packages/dashboard/app/components/InlineCreateCard.tsx

@@ -767,7 +767,7 @@ export function InlineCreateCard({
                   data-testid="plan-button"
                   title="Open planning mode with current description"
                 >
-                  <Lightbulb size={12} style={{ verticalAlign: "middle" }} />
+                  <Lightbulb size={12} style={{ verticalAlign: "middle", marginRight: 4 }} />
                   Plan
                 </button>
                 <button
@@ -778,7 +778,7 @@ export function InlineCreateCard({
                   data-testid="subtask-button"
                   title="Break down into AI-generated subtasks"
                 >
-                  <ListTree size={12} style={{ verticalAlign: "middle" }} />
+                  <ListTree size={12} style={{ verticalAlign: "middle", marginRight: 4 }} />
                   Subtask
                 </button>
                 <button