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/java/com/m57/hermescontrol/data/model/SessionListResponse.kt

@@ -23,6 +23,9 @@ data class SessionInfo(
     val display_name: String? = null,
     val model: String? = null,
     val terminal_backend: String? = null,
+    // Durable "keep" flag: pinned sessions are exempt from the auto-archive
+    // sweep and are surfaced first in the history list (sorted client-side).
+    val pinned: Boolean? = null,
 )
 
 @Serializable
@@ -35,7 +38,10 @@ data class SessionStatsResponse(
 
 @Serializable
 data class SessionRenameRequest(
-    val title: String,
+    // Both fields are optional: the backend PATCH accepts any subset
+    // (SessionRename model: title/archived/pinned, omitted = unchanged).
+    val title: String? = null,
+    val pinned: Boolean? = null,
 )
 
 @Serializable