fix(desktop): recover from proxy sidecar exits

[kev1n77]

Summary

• poll the verified proxy sidecar without blocking and detect unexpected process exit
• isolate monitors by runtime generation so stale tasks cannot stop a replacement session
• remove the failed proxy runtime, restore the system proxy, re-enable Gateway capture, and emit a structured runtime error
• reload the frontend workspace after sidecar failure so the displayed mode matches the recovered backend state
• update CAP-006 progress and threat-model evidence

Verification

• cargo fmt --all -- --check
• cargo test --workspace --all-targets
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --manifest-path apps/desktop/src-tauri/Cargo.toml (12 unit + 1 watchdog integration)
• cargo clippy --manifest-path apps/desktop/src-tauri/Cargo.toml --all-targets -- -D warnings
• npm run check (20 tests)
• npm run build