Add org switcher with create, switch, and delete

The sidebar org button was inert: clicking it did nothing. Add a real
dropdown menu listing every organization, a "New organization" entry
that re-enters onboarding without resetting the database, and a per-org
delete affordance with inline confirmation. Active org is now persisted
in a single-row app_state table so the snapshot keeps a stable
selection across restarts and the renderer drives switching via two new
IPC channels (setActiveOrganization, deleteOrganization).

- migration 006 adds app_state(id=1, active_org_id, updated_at)
- snapshot resolves active org from app_state with the first-org fallback
- OrganizationService gains setActive + delete + bootstrap upsert
- new OrgSwitcher renderer component keeps Shell.tsx lean

AgentCompany-New/src/preload/index.ts

@@ -10,6 +10,8 @@ const api: DesktopApi = {
   loadSnapshot: () => invoke(IPC_CHANNELS.loadSnapshot),
   bootstrapOnboarding: (input) => invoke(IPC_CHANNELS.bootstrapOnboarding, input),
   createOrganization: (input) => invoke(IPC_CHANNELS.createOrganization, input),
+  setActiveOrganization: (input) => invoke(IPC_CHANNELS.setActiveOrganization, input),
+  deleteOrganization: (input) => invoke(IPC_CHANNELS.deleteOrganization, input),
   createAgent: (input) => invoke(IPC_CHANNELS.createAgent, input),
   createTask: (input) => invoke(IPC_CHANNELS.createTask, input),
   updateTaskStatus: (input) => invoke(IPC_CHANNELS.updateTaskStatus, input),