[Agent] New post form breaks when project has zero boards

Agent brief

Category: bug | Severity: medium | Effort: S

Problem

When project has zero feedback boards, /feedback/new still renders form; submit fails with confusing error.

Repro

1. New project with no boards configured
2. Visit /feedback/new
3. Form renders but cannot submit successfully

Files

• src/app/[projectSlug]/feedback/new/page.tsx
• src/components/feedback/new-post-form.tsx

Acceptance criteria

• Empty boards → friendly empty state with admin hint
• Submit disabled when no board selected

Suggested approach

Early return empty state when boards.length === 0.