F8.2d: Add drag-and-drop parity and repeatable PWA file tests

[Flow-Fly]

Parent

• F8.2 — OS file-handler registration for .pf / .aseprite #123

Goal

Give browser users the same safe project-import path through drag-and-drop, and make local PWA/file-handler verification easy to repeat.

Context

File handling is desktop Chromium-only. File menu and drag-and-drop remain the cross-browser paths and should share identical parsing, project-creation, capacity, and error behavior.

Risk and routing

Risk: low
Readiness: ready-for-agent

This adds adapters around an already-tested import service and a local preview command; it does not broaden the supported file formats.

Acceptance Criteria

• Dropping supported project files anywhere on the app imports them sequentially through the shared service.
• Unsupported drops are not intercepted.
• Failures and tab-limit outcomes produce concise, polite, non-modal feedback.
• File-menu import uses the same service.
• A pwa:preview script builds and serves the production PWA locally.
• The verification guide covers install/update, OS double-click, multi-file launch, drag-and-drop, permissions, and non-supporting browsers.

Suggested Files

• src/components/app/pixel-forge-app.ts
• src/components/menu/pf-menu-bar.ts
• package.json
• docs/verification/pwa.md
• focused component/service tests

Verification

• Focused drag/drop and File-menu tests.
• npm run test:run
• npm run build
• git diff --check
• Manual installed-PWA/Finder proof documented in the PR.

Dependencies

• Uses the shared import and launch handling implemented by the preceding child issues.

Out of Scope

• Image/reference drag-and-drop.
• A general notification framework.
• Direct source-file editing.

[Flow-Fly]

Agent Assessment

Risk: low
Agent-ready: yes
Routing: ready-for-agent

Evidence:

• File menu and drop adapters can delegate to the same public import service.
• Supported-file filtering prevents interference with image/reference drops.

Why this is safe:

• No new parsing path or format; feedback is non-modal and the preview script only runs a production build locally.

Verification:

• Component adapter tests, full tests, production build, diff check, and documented manual proof.

Human question or blocker:

• None for implementation. Finder double-click remains a final playtest check.

[Flow-Fly]

Implemented in #334 (commits c38ac83 and b7914a2). File-menu parity, protected-mode drag/drop, feedback, and the repeatable preview guide are covered. Awaiting the parent playtest and merge gate.