[Agent] Invalid ?board= slug silently ignored on new post

Agent brief

Category: bug | Severity: low | Effort: XS

Problem

?board=invalid-slug on /feedback/new is silently ignored; form defaults to first board without warning.

Repro

1. Visit /feedback/new?board=nonexistent
2. Form opens with wrong board pre-selected

Files

• src/app/[projectSlug]/feedback/new/page.tsx line 28

Acceptance criteria

• Invalid board slug shows inline warning or 404
• Valid slug pre-selects correctly

Suggested approach

If board param present but not found, show toast or banner.