Add global synthesis and terminal settings

[milanshen]

Context

coSlash currently hard-codes its synthesis backend, model, and macOS terminal. This adds secure machine-wide settings with explicit synthesis consent, configurable runtime choices, and fail-closed handling for invalid configuration.

Changes

• Add a versioned ~/.coslash/settings.json schema and restricted atomic persistence.
• Add first-run synthesis consent and a full settings UI with availability and error states.
• Support Claude Code and Codex synthesis backends with selectable models.
• Support Apple Terminal and iTerm2 with availability checks and immediate runtime updates.

Test

fresh start or delete ~/.coslash/settings.json

Screenshots

[luhe19001]

Requested change

First-run can save synthesis On with an unavailable backend still selected.

The consent draft defaults to enabled: true and claude-cli. Unavailable backends are not clickable, but the current selection is not cleared and Save is not blocked — so a Codex-only machine can accept the dialog as-is and persist a config that fails every synthesis run.

Please block that path, e.g. auto-select the first available backend, disable Save when synthesis is on and the selected backend is unavailable, or default draft enabled to false when no backend is available.