Two hand-rolled copies of the add-on/brainMode rule replaced by a single shared helper, and the one-line isCompanyBrainOrganization wrapper dropped. No behaviour change.
## What changed
- Make `/brain` reuse the active Company Brain, switch to a single existing Company Brain, show a picker for multiple choices, or create one only when none exists.
- Wait for active-organization restoration before making that decision.
- Make Company Brain onboarding match organizations by the confirmed company domain and never offer unrelated-domain workspaces.
- Create a new Company Brain when no matching workspace exists and show an actionable workspace-limit toast when creation is blocked.
- Improve the organization picker, loading, and error states.
## Why
The previous flows could use or mutate the currently active normal organization, start research against stale Company Brain metadata, or offer unrelated Company Brain workspaces after a quota failure.
## Impact
Normal organizations are no longer silently converted. Research is scoped to the Company Brain for the confirmed domain, and users receive a clear recovery path when they reach their workspace limit.
This keeps the existing `{ domain }` research API contract; no organization-reconfiguration API is required.
## Validation
- Biome checks passed for all five changed web files
- Targeted web TypeScript diagnostics reported no errors for the changed files
- React Doctor against `origin/main`: no issues found
- `git diff --check`
No test files were added.
