feat: multi-session management (browse all + bind N sessions per notebook)

User can now see EVERY session on the OpenCode Serve side (not just
the per-notebook mapping) and bind multiple sessions to a single
notebook. One of the bound sessions is the "active" one (the one
the next /edit uses); the others are historical. The user can
switch, create, or delete via the new session selector UI in the
inline prompt.

Backend
-------
- SessionManager rewritten for 1-notebook-N-sessions: keeps both an
   map and a  map. New methods: create_new,
  bind_existing, set_active, unbind, unbind_active, delete_session,
  list_sessions_for_notebook. Existing get_or_create / peek /
  invalidate / list_sessions / release semantics preserved
  (release now deletes ALL bound sessions, not just the active).
- New route GET /opencode-bridge/sessions/all — list ALL OpenCode
  sessions via /session.
- New route GET /opencode-bridge/sessions/notebook?notebook=... —
  bound sessions for one notebook, enriched with title/createdAt
  from the global list.
- POST .../sessions/notebook — create a new session, bind, set
  active.
- PUT  .../sessions/notebook — bind an existing sessionId, set
  active (no OpenCode round-trip).
- DELETE .../sessions/notebook — delete the session on OpenCode
  AND remove its binding.
- New route GET/PUT/DELETE /opencode-bridge/sessions/active —
  read / switch / unbind the active session for one notebook.
- OpenCodeClient.list_all_sessions() — proxy OpenCode /session.

Frontend
-------
- SessionSelector dropdown in the inline prompt: lists ALL OpenCode
  sessions with the active one selected, plus a "+ 新建会话"
  sentinel entry. "+ 新建" button + "🔄" refresh button.
  Switching calls onSwitchSession; selecting "+ 新建会话" or
  clicking the new button calls onCreateSession. After both,
  the prompt's history is reloaded for the new active session.
- New cell-action callbacks: onListSessions, onCreateSession,
  onSwitchSession, onReloadHistory. Each closes the SSE first
  so events for the OLD session don't bleed in during the switch.
- 8 new API client functions in api/opencode_client.ts.
- New types in types.ts: OpenCodeSessionMeta,
  OpenCodeNotebookSession, OpenCodeAllSessionsResponse,
  OpenCodeNotebookSessionsResponse, OpenCodeSessionOpResponse.

Tests
-----
- 73 backend pytest (was 64): +6 new SessionManager multi-session
  unit tests + 7 new route tests (all-sessions, notebook GET/POST/
  PUT/DELETE, active GET/PUT/DELETE).
- 76 frontend jest (was 72): +4 new prompt tests (session selector
  rendered when callbacks wired, initialize() populates the list,
  + 新建 creates+binds, dropdown change switches and reloads).
- package.json version bumped 0.1.0 -> 0.1.1 to match the v0.1.1 tag
  that was already published.

Co-Authored-By: Claude <noreply@anthropic.com>

style/base.css

@@ -339,3 +339,52 @@
 .opencode-inline-prompt .opencode-q-submit:hover {
   background: var(--jp-layout-color3, #e0e0e0);
 }
+
+/* ------------------------------------------------------------------
+   Session selector (multi-session UI) — top of the inline prompt
+   ------------------------------------------------------------------ */
+.opencode-inline-prompt .opencode-prompt-sessions {
+  display: flex;
+  gap: 6px;
+  align-items: center;
+  flex-wrap: wrap;
+  margin: 2px 0 4px;
+}
+.opencode-inline-prompt .opencode-prompt-sessions label {
+  display: flex;
+  align-items: center;
+  gap: 4px;
+  font-size: var(--jp-ui-font-size1);
+  color: var(--jp-ui-font-color1, #333);
+  flex: 1;
+  min-width: 0;
+}
+.opencode-inline-prompt .opencode-session-select {
+  flex: 1;
+  min-width: 0;
+  padding: 2px 4px;
+  font-size: var(--jp-ui-font-size1);
+  font-family: inherit;
+  border: 1px solid var(--jp-border-color2, #ccc);
+  border-radius: 3px;
+  background: var(--jp-layout-color1, #fff);
+}
+.opencode-inline-prompt .opencode-session-btn-new,
+.opencode-inline-prompt .opencode-session-btn-refresh {
+  border: 1px solid var(--jp-border-color2, #ccc);
+  background: var(--jp-layout-color2, #f0f0f0);
+  padding: 2px 8px;
+  border-radius: 3px;
+  cursor: pointer;
+  font-size: var(--jp-ui-font-size0, 11px);
+  color: var(--jp-ui-font-color1, #333);
+}
+.opencode-inline-prompt .opencode-session-btn-new:hover,
+.opencode-inline-prompt .opencode-session-btn-refresh:hover {
+  background: var(--jp-layout-color3, #e0e0e0);
+}
+.opencode-inline-prompt .opencode-session-btn-new {
+  background: #dbeafe;
+  color: #1e40af;
+  border-color: #93c5fd;
+}