Vouch-gated role application on join page

[hudsonhrh]

Summary

Detect when all organization roles require vouching and integrate role application into the join flow. Users now select a role, fill out an application (why they want it, relevant experience), join the org, and apply for the role in a single coherent experience.

Changes

• RoleApplicationForm: New reusable component with role selector, notes (required), and experience (optional) fields
• Join page detection: Reads org roles via useOrgStructure; when all roles have vouchingEnabled=true and defaultEligible=false, switches to "Apply & Join" flow
• handleApplyAndJoin handler: Joins via quickJoin, then applies via applyForRole (IPFS upload + contract call)
• Loading guard: Shows spinner while org data fetches to prevent UI flashing
• Auto-select: Single-role orgs auto-select the role on first render
• Backward compatible: Existing join flow unchanged when roles are freely claimable

Testing

1. Deploy org with all roles requiring vouching
2. Visit /user/?userDAO=<orgName> → "Apply to Join" messaging appears
3. Authenticate → role selector + application form shown
4. Fill form → click "Apply & Join" → verify join + application both succeed
5. Verify application appears in org-structure VouchingSection for members to vouch
6. Test org with freely claimable roles → existing join flow unchanged