F8.1: Make Pixel Forge installable and offline-ready

[Flow-Fly]

What changed

• Added a standalone PWA manifest, branded icons, and a Workbox service worker that precaches the application shell.
• Added a quiet Install Pixel Forge action to the File menu. It only appears while the browser exposes an install prompt.
• Added a persistent, non-modal update notice with Later and Restart actions.
• Made restart save the active project before activating a waiting service worker. Save or update failures keep the current session open.
• Added a repeatable install, offline, and A/B update verification guide.

Why

Pixel Forge is local-first, so installability and offline startup are useful even before richer PWA integrations arrive. The prompt-controlled update flow protects active drawing sessions from surprise reloads or code swaps.

Verification

• npm run test:run — 91 files, 504 tests passed
• npm run lint — no errors; 5 existing no-explicit-any warnings
• npm run build — manifest and service worker generated, 60 entries precached
• git diff --check
• Browser proof: after loading once, the complete editor shell reloaded with the preview server stopped
• A/B browser proof: version A stayed active until version B produced the Update ready notice; there was no automatic reload

Manual merge gate

Follow the PWA verification guide in Chromium to confirm the browser-owned install prompt, standalone launch, an offline drawing save/reload, and both update actions.

Closes #122
Closes #326
Closes #327
Closes #328

#123 remains the separate follow-up for OS file handling.