[P1] Prevent sticky unsaved-changes guard across sessions

[kgarg2468]

Problem

isDirty is persisted in localStorage, so users can get blocked by unsaved-changes modal after reload/new session without current edits.

Evidence: lib/store/business-store.ts persists isDirty via partialize.

Scope

• Stop persisting ephemeral dirty-state or reset it safely on hydration/route entry.

Acceptance Criteria

• Fresh page load does not show unsaved-changes block unless edits happened in current session.
• Project switching only prompts when there are real unsaved edits.

Suggested Files

• lib/store/business-store.ts
• app/(app)/data/page.tsx