F1 follow-up — Do not auto-open Project Library on first run

[Flow-Fly]

Problem

When Pixel Forge starts without a saved workspace or project, it automatically opens the Project Library as a non-dismissible first-run gate. This slows repeated local testing and is not yet a deliberate onboarding experience.

Scope

• Start in the editor when no saved project can be restored.
• Keep explicit Project Library entry points available through File/Open Project, the project tab strip, and the existing shortcut.
• Keep the Project Library dismissible when it was opened explicitly from a fresh editor.
• Continue requiring a project choice after the active persisted project is deleted.
• Do not redesign onboarding in this issue.

Acceptance

• Empty-storage startup renders the editor without opening Project Library.
• Explicitly opening Project Library from that state can be dismissed.
• Canceling New Project or Guided Drawing returns to the editor when selection is optional.
• Deleting the active persisted project still opens a non-dismissible Project Library.
• Focused tests cover startup and required/optional selection behavior.

[Flow-Fly]

Agent Assessment

This is a low-risk, bounded F1 UX follow-up.

• The editor already has a valid transient project when persistence is empty.
• The change is confined to startup/project-selection state in pixel-forge-app plus focused component tests.
• Explicit Project Library entry points remain unchanged.
• Deleting the active persisted project remains a required-selection safety path.
• No onboarding redesign, persistence schema, or project-library service change is included.

Verification: 486 tests, lint, build/typecheck, Fallow, and git diff --check pass locally.

[Flow-Fly]

Completed in #321 and verified on develop at 6c765e2 by CI run 29088835453.