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/main/database/migrations/006-app-state.ts

@@ -0,0 +1,14 @@
+export const appStateSchema = `
+  create table if not exists app_state (
+    id integer primary key check (id = 1),
+    active_org_id text references organizations(id) on delete set null,
+    updated_at text not null
+  );
+
+  insert into app_state(id, active_org_id, updated_at)
+  select 1, (select id from organizations order by created_at asc limit 1), datetime('now')
+  where not exists (select 1 from app_state where id = 1);
+
+  insert into schema_migrations(version, name, applied_at)
+  values (6, 'app_state', datetime('now'));
+`;