Fix task delete cascade, multi-company UI, and error handling

- RequestDetail: remove auto-project-delete when last task deleted
- MyCompany: support multiple companies with selector dropdown
- MyCompany: fetch members from both profiles and company_members
- ProjectDetail/MyProjectDetail/InvoiceDetail: check Supabase errors before updating state
- Migration: drop client project-delete RLS policy (no longer valid)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

supabase/migrations/20260513150000_fix_client_project_delete_policy.sql

@@ -0,0 +1,4 @@
+-- Remove the client project-delete policy.
+-- It was added to support auto-deleting empty projects when a client deletes their
+-- last task, but that behavior is wrong — projects should only be deleted explicitly.
+drop policy if exists "Client deletes company projects" on public.projects;