Fix custom Default presets in local tasks

[ilysenko]

Summary

• make configured Default presets replace upstream slider positions in local repository tasks as well as cloud/TPP chats
• keep explicit model selection behavior unchanged
• add fail-closed semantic coverage and a local-mode runtime regression fixture

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)
• latest signed stable 26.903.71938 configured ASAR patch
• official Linux feature audit (33 features)
• git diff --check

Notes

The previous implementation used the configured slider exclusively only when isTppConversation was true. Local tasks merged upstream power positions back in, producing the old six-position slider.