Fix idle account recovery and add default prompt toggle

[xiaodream551-a11y]

Summary

• add a Global System Prompt option to disable the built-in Antigravity identity prompt while keeping custom global prompts available
• apply the default prompt toggle across Claude, OpenAI, and Gemini-native request mappers
• reset account health/rate-limit state using account_id on successful requests and reload the account cache once before returning stale "All accounts failed or unhealthy" errors

Fixes #3082
Fixes #945

Tests

• cargo test --manifest-path src-tauri/Cargo.toml -p antigravity_tools test_default_system_prompt_can_be_disabled -- --test-threads=1 --nocapture
• cargo test --manifest-path src-tauri/Cargo.toml -p antigravity_tools test_mark_account_success_accepts_email -- --nocapture
• cargo test --manifest-path src-tauri/Cargo.toml -p antigravity_tools test_stale_token_pool_errors_trigger_reload -- --nocapture
• npm run build