Add missing getAvailableShells stub to session-save test mock

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: nedtwigg

lib/src/lib/session-save.test.ts

@@ -24,6 +24,7 @@ function createPlatform(savedState: PersistedSession | null): PlatformAdapter {
     writePty: () => {},
     resizePty: () => {},
     killPty: () => {},
+    getAvailableShells: vi.fn(async () => []),
     getCwd: vi.fn(async () => '/tmp/live'),
     getScrollback: vi.fn(async () => 'echo hello\n'),
     onPtyData: () => {},