fix(mobile): hide screen sharing in native webviews

[ScottShao]

Related to #13
Related to #1
/claim #1

Summary

• detect Capacitor native app context in the front-end via the existing browser/device utility layer
• hide browser screen sharing when Universe is running inside the Android/iOS Capacitor shell so mobile users do not get a broken getDisplayMedia path
• document screen sharing as the expected browser-only limitation in mobile/README.md while leaving camera/mic and iframe behavior unchanged
• add a focused Vitest file that locks the native-app detection and screen-share support decision logic

Why this slice

This is a narrow follow-up from #13, not the full WebView compatibility issue. The goal is to handle the known mobile screen-share limitation cleanly without overlapping the already-open Android/iOS platform PRs (#21 and #22), the push work in #11, or the update flow in #12.

Verification

• git diff --check
• npm exec --workspace play -- prettier --check src/front/WebRtc/DeviceUtils.ts src/front/WebRtc/DeviceUtils.test.ts src/front/Stores/ScreenSharingStore.ts

Environment limitations

• npm test --workspace play -- DeviceUtils.test.ts is currently blocked by the repo's Vite/Svelte setup on universe (vite.config.mts imports sveltePreprocess as a named export from the CommonJS svelte-preprocess package)
• npm run typecheck --workspace play is also already failing on the current universe branch because generated @workadventure/messages protobuf outputs and related imports are missing from the workspace checkout
• npm exec --workspace play -- eslint ... still reports the pre-existing import/no-unresolved problem for ../../i18n/i18n-svelte inside ScreenSharingStore.ts; this resolver issue exists on the branch independently of this slice

Demo

This slice has no visual asset change or native build artifact. The user-facing behavior change is that the screen-share control is no longer advertised inside the Capacitor mobile shell, and the README now documents that screen sharing remains browser-only on mobile.

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⚙️ Run configuration

Configuration used: defaults

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Run ID: c32dab44-163c-4376-b80c-018a59ff82c1

📥 Commits

Reviewing files that changed from the base of the PR and between e3106e7 and e538b77.

📒 Files selected for processing (4)

• mobile/README.md
• play/src/front/Stores/ScreenSharingStore.ts
• play/src/front/WebRtc/DeviceUtils.test.ts
• play/src/front/WebRtc/DeviceUtils.ts

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