Add a 3-state theme toggle in folder selection and instance tabs, and update tokens/styles so light mode has readable contrast. Sync MUI surfaces and Shiki highlighting to CSS variables to prevent stale colors when switching themes.
Adds an optional session filter bar to the left sidebar with title search across parent/child sessions and a scoped Select All. Introduces multi-select checkboxes, bulk delete with clear selection controls, and confirmation dialogs for both single and bulk deletions using the existing alert dialog flow. Updates session i18n strings across supported locales.
Converts hardcoded UI copy to i18n keys across the app, adds global translation for non-component modules, and splits the English catalog into feature modules with duplicate-key detection.
