Keep the installer's proxy and clean up node uninstall

[SaladDay]

What

• Node service keeps the proxy settings the installer ran with: /etc/oac-node/<id>.env (root, 0600) read through EnvironmentFile=- in the unit. Both spellings, localhost,127.0.0.1,::1 appended to NO_PROXY. A rerun replaces the file (and try-restarts a running service when it changed); a run without an HTTP(S) proxy removes it; uninstall removes it. Credentials never enter the world-readable unit. The child's proxy filter is now the shared proxy_settings.
• Microsandbox uninstall deletes the node's store (~/.oac/m/<sha12>, only when its oac-installation.json names this installation) once Core confirmed the removal, so the created account is released in the same run. --force (Core not asked) keeps the store and prints the exact rm -rf command. Account deletion still depends on account.json created. Docker image hint unchanged; the output says deleted vs kept. The node_spec.fetch uninstall hint (Lane H) is untouched.
• Docs (EN + zh, source_hash refreshed): proxy section, Records row, Connection failures row, uninstall paragraph; README rules.

Why

audit-node finding 6 (proxy-only hosts) and 9 (uninstall multi-step); decisions.md decision 4. No new flag or setting.

Verification

• python3 -m unittest test_node_proxy, test_node_install, test_node_spec, test_node_readiness, test_node_generations, test_generation_review_regressions, test_distribution, test_node_helper_transfer: pass
• scripts/check-names.py: pass; translations test 1-2 pass, test 3 needs vitepress (fails identically on main here), skipped.

Left for live check on mx2 (root)

Install behind a proxy and check the 0600 env file and journal connect; rerun with another proxy; microsandbox uninstall frees ~/.oac/m and the account.

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