docs: update package and module READMEs

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Summary

Updated package/module README files to improve developer guidance.

Changes

• Updated existing READMEs: 7
• Created missing READMEs: 0
• Skipped already up-to-date READMEs: 28
• Skipped due to insufficient evidence: 0

Validation

• Verified README content against source code
• Verified Markdown formatting
• Verified examples reference real APIs
• Verified only README files were changed

Notes

• camera: Documented starter hasCamera enable script, useCamera registration, web image_worker.js, and permissions added by the script.
• face_camera: Documented starter hasFaceCamera enable script, useFaceCamera registration, and web face-detection scripts.
• qr_scanner: Corrected module comparison table — face detection is in ensemble_face_camera, not ensemble_camera.
• device_preview: Added Ensemble monorepo context and pointer to packages/ensemble_device_preview.
• ensemble_icons / ensemble_walletconnect: Updated dependency version pins to match pubspec.yaml.
• flutter_share_receiver: Replaced deprecated FlutterApplication with ${applicationName} to match the package example.

Skipped packages (28) were reviewed and deemed accurate against current source. modules/ensemble_ts_interpreter is excluded per melos.yaml ignore rules.