Add a selection mode to the HistoryPanel that lets users select multiple
sessions and delete them in one action. The active session is never
selectable (shown with a '—' indicator).

Backend: Add DeleteSessions(paths) Go method that iterates paths, skips
the active session, and deletes each via deleteSessionFile. Returns the
count of successfully deleted sessions.

Frontend: Selection mode toggles via a Select/Done button in the drawer
header. Each non-current session row shows a checkbox; a footer bar
provides select-all toggle and a delete-selected button with two-step
confirmation (same pattern as single delete). In selection mode, clicking
a row toggles the checkbox instead of resuming the session.

Bridge + Controller: Added DeleteSessions binding and deleteSessions
callback. Mock implementation for browser dev mode included.

i18n: Added 6 keys for selection mode in en.ts and zh.ts (select, done,
select all, deselect all, delete selected, confirm delete selected).

CSS: Added styles for checkboxes, selected highlight, footer action bar,
generic button variants, and active chip state.