feat(FN-2391): add clickable per-file diffs in Git Manager

- Add a new /api/git/diff/file endpoint with path and staged query handling, including validation and untracked-file diff support
- Introduce fetchGitFileDiff API client wiring and update GitManagerModal to load diffs for selected staged/unstaged file rows
- Improve file-row UX with active/focus styling, keyboard activation, and guarded async diff loading/error states
- Expand dashboard route and modal tests to cover per-file diff behavior, query validation, and stage/unstage interaction isolation

packages/dashboard/app/components/ScriptsModal.css

@@ -2019,6 +2019,7 @@
   border-bottom: 1px solid rgba(255, 255, 255, 0.03);
   font-size: 13px;
   transition: background var(--transition-fast);
+  cursor: pointer;
 }
 
 .gm-file-item:last-child {
@@ -2033,6 +2034,15 @@
   background: rgba(63, 185, 80, 0.03);
 }
 
+.gm-file-item.active {
+  background: var(--card-hover);
+}
+
+.gm-file-item:focus-visible {
+  outline: none;
+  box-shadow: var(--focus-ring-strong);
+}
+
 .gm-file-checkbox {
   display: flex;
   align-items: center;
@@ -2105,6 +2115,20 @@
   gap: var(--space-sm);
 }
 
+.gm-diff-target {
+  display: flex;
+  align-items: center;
+  gap: var(--space-xs);
+  font-size: 12px;
+  color: var(--text-muted);
+}
+
+.gm-diff-target code {
+  font-family: var(--font-mono);
+  font-size: 11px;
+  color: var(--text);
+}
+
 .changed-files-diff-section {
   height: 100%;
 }