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/QuickEntryBox.tsx

@@ -597,7 +597,7 @@ export function QuickEntryBox({ onCreate, addToast, tasks = [], availableModels,
                 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
@@ -609,7 +609,7 @@ export function QuickEntryBox({ onCreate, addToast, tasks = [], availableModels,
                 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
@@ -621,7 +621,7 @@ export function QuickEntryBox({ onCreate, addToast, tasks = [], availableModels,
                 data-testid="save-button"
                 title="Create task"
               >
-                <Save size={12} style={{ verticalAlign: "middle" }} />
+                <Save size={12} style={{ verticalAlign: "middle", marginRight: 4 }} />
                 Save
               </button>
               <div className="refine-trigger-wrap" ref={refineMenuRef}>