Fix custom model context-window overflow recovery

[zhouzhuozhzh-maker]

Summary

• add persistent per-model context-window overrides for custom and proxied models
• merge overrides with curated metadata so the context meter and compaction engine share one effective value
• reserve response/safety headroom when retaining recent history and hard-bound recovery summarization to the selected model's window
• add Settings UI plus backend, engine, pure-policy, and Playwright coverage for a 32,768-token custom model

Verification

• .venv/bin/python -m pytest -q — 1116 passed, 1 skipped
• npm test — 108 passed
• npm run build
• npm run e2e -- e2e/compaction.spec.ts — 2 passed

Fixes #441

[zhouzhuozhzh-maker]

Screenshot

Models → Context compaction with a custom 32,768-token window: