feat: add language switcher in TUI settings

[corentin-core]

Context

Issue #28 introduces i18n support with gettext (English default + French translation).
Currently the language is configured via config.yaml. Users should be able to switch
language directly from the TUI without editing the config file.

Proposed solution

• Add a settings screen or modal accessible from the TUI
• Allow selecting language (en/fr) from a dropdown
• Persist the choice to config.yaml
• Apply the change immediately (or on next restart)

Acceptance criteria

• Language can be changed from the TUI interface
• Choice is persisted to config.yaml
• Change takes effect without manual config editing

Related issues

• Depends on feat: multi-language support (i18n) with English #28 (i18n infrastructure)