Add PWA file-handling guide and launch support

[zhaobozhen]

Summary

• Add a desktop PWA install guide that explains how to install LibChecker WebUI and open APK, APKS, APKM, and XAPK files from Finder or File Explorer.
• Register Chromium File Handling API associations and route launchQueue files through the existing local analyzer flow.
• Add the web app manifest, install icons, localized copy, and lazy build handling required for desktop PWA integration.
• Refine the auto-replaying SVG walkthrough with the LibChecker mark, Chrome-style install control, measured label width, staged checkmark drawing, reduced-motion behavior, and low-power fallback.
• Add manifest, file-handler, browser interaction, accessibility, and motion regression coverage.

Why

The WebUI previously had no system-level entry point for installed desktop users and no in-product explanation of the install-and-share workflow. The guide appears after a successful local analysis, when the benefit is relevant, while retaining picker and drag-and-drop fallbacks for browsers without file handling support.

The animation also used fixed control geometry and guessed stroke lengths, which caused excess padding and an early checkmark fragment. Localized text width and SVG path length are now measured at runtime with compact fallbacks.

Validation

• npm run deploy:preflight -- --target=preview
• npm run pages:check
• npm run test:browser --workspace @tgbot/apk-webui
• npm run test:browser --workspace @tgbot/apk-webui -- test-browser/pwa.spec.mjs
• npm run perf:check

All preview preflight and performance budget rows passed.

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