feat(history): pin sessions to the top of the history tab (#886)

- Hold-menu Pin/Unpin action on session cards (PATCH /api/sessions/{id}
  with {pinned}, verified against the backend SessionRename model)
- Pinned sessions sort to the top on every load (client-side stable sort
  — the backend back-fills pins into page 1 but does not lift them)
- Pin indicator on pinned cards; search-result cards omit the pin action
- Tests: pin-first sort on load, toggle success (moves to top), failure
  (order kept + toast)

Co-authored-by: M57 <hy4ri@users.noreply.github.com>

app/src/main/res/values/strings.xml

@@ -543,6 +543,9 @@
     <string name="sessions_action_delete_n">Delete %1$d</string>
     <string name="sessions_action_select">Select</string>
     <string name="sessions_action_rename">Rename</string>
+    <string name="sessions_action_pin">Pin</string>
+    <string name="sessions_action_unpin">Unpin</string>
+    <string name="sessions_pinned_indicator">Pinned</string>
     <string name="sessions_rename_title">Rename Session</string>
     <string name="sessions_rename_hint">Session title</string>