Introduce PrivacyError and harden privacy service listeners

[RAprogramm]

Summary

• add a typed PrivacyError and wire it through the privacy service state machine, listeners, and UI consumer
• gracefully degrade when the webcam device is absent and surface initialization failures instead of panicking
• extend idle inhibitor tests and tighten brightness logging to satisfy clippy
• bump the crate version to 0.2.1 and document the release

Testing

• cargo +nightly fmt --
• cargo +nightly clippy -- -D warnings
• cargo +nightly test --all
• cargo +nightly build --all-targets
• cargo +nightly doc --no-deps
• cargo audit
• cargo deny check (fails: advisory-db fetch blocked in environment)

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https://chatgpt.com/codex/tasks/task_e_68d60b0b1250832baf75e1b928464973