feat(dashboard): move org switcher from sidebar to header

[aaight]

Summary

• Moved the OrgSwitcher component from the sidebar into the header bar, replacing the static org-name Badge with an interactive <Select> dropdown for admin users with multiple orgs
• For single-org admins, shows the org name as plain text (preserving previous context visibility)
• Cleaned up sidebar.tsx by removing the OrgSwitcher function, its usage, and all now-unused imports (Select, SelectContent, SelectItem, SelectTrigger, SelectValue, Building2, useOrgContext)

Test plan

• Log in as admin with multiple orgs → org switcher dropdown appears in top nav header (left side)
• Switch org via dropdown → all data refreshes correctly (same behavior as before)
• Log in as admin with single org → org name shown as text in header, no dropdown
• Log in as non-admin → no org switcher or org name in header
• Verify sidebar no longer shows the org switcher section (nav flows directly after CASCADE branding)
• npm run lint and npm run typecheck pass ✅

Card: https://trello.com/c/FtarOIgX/34-lets-move-the-admins-organization-switcher-to-top-nav-replacing-word-admin-we-have-there

🤖 Generated with Claude Code

[nhopeatall]

🔍 Reviewing PR...

[nhopeatall]

LGTM — clean relocation of the org switcher from sidebar to header. The conditional logic correctly handles all three user states (multi-org admin → dropdown, single-org admin → plain text, non-admin → nothing). Sidebar cleanup removes all now-unused imports. No issues found.