[v1.0] PWA polish — installable, offline, app icons

[Jonathangadeaharder]

PWA polish

(Folds in / supersedes #225 "Add PWA support".)

Goal

Tilt is installable on iOS/Android home screens; once installed, it loads offline and feels like a native app.

Tasks

• manifest.webmanifest with full icon set (192, 512, maskable, monochrome)
• App icons exported from the SVG source (script under scripts/build-icons.ts)
• Service worker (via @vite-pwa/sveltekit) precaches the app shell + last N SRS cards for offline review
• "Install Tilt" prompt on second visit (deferred via beforeinstallprompt)
• Splash screen matches brand
• Offline page when route is not cached
• Lighthouse PWA audit ≥95

Acceptance

Lighthouse PWA score ≥95 on /. Adding to home screen on iOS shows the Tilt icon and launches in standalone mode.