Fix custom Default effort resolution

[ilysenko]

Summary

• resolve configured effort variants for account-visible models instead of requiring each pair in the server recommended set
• expose private resolver options without changing the manual model list
• keep upstream workspace and Ultra/XHigh gates, and document the corrected fallback behavior

Tests

• node --test linux-features/model-picker-default-presets/test.js
• node --test scripts/patch-linux-window-ui.test.js scripts/lib/linux-features.test.js linux-features/*/test.js (907 passed, 1 skipped)
• official Linux feature audit against signed 26.903.61454 ASAR
• isolated configured patch against signed 26.903.61454 ASAR
• git diff --check

Notes

• Manual model options remain upstream-owned; only the Default slider receives synthesized resolver entries for configured efforts.