fix: searchable client combobox in Nova instalação

Replaces the native list with a combobox (name/code/CNPJ search, accent- and
case-insensitive, keyboard + mouse, custom dropdown, hidden client_id = the
Gerenciador's real CLIENTE.ID). The backend resolves client_id against the
active GTI_GERENCIADOR mirror and rejects free text/unknown ids. Adds
Tenant.external_cnpj (migration 0025) mirrored via the internal endpoint.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

central/alembic/versions/0025_tenant_external_cnpj.py

@@ -0,0 +1,20 @@
+"""Tenant.external_cnpj (digits) mirrored from GTI Gerenciador for the client selector search.
+
+Revision ID: 0025_tenant_external_cnpj
+Revises: 0024_tenant_gerenciador_mirror
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "0025_tenant_external_cnpj"
+down_revision = "0024_tenant_gerenciador_mirror"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.add_column("tenants", sa.Column("external_cnpj", sa.String(20), nullable=True))
+
+
+def downgrade():
+    op.drop_column("tenants", "external_cnpj")