feat(chat): configurable full-width chat layout

[touch2be]

Summary

• Add a persisted Standard / Full width option under Settings → General → Appearance.
• Apply the same width choice to the transcript and prompt area while preserving side gutters and table-local horizontal scrolling.
• Add translations (DE/EN/ES) and tests for preference persistence and transcript layout switching.

Validation

• vp test src/hooks/use-chat-width.test.tsx src/features/session-transcript/transcript-viewport.test.tsx (9 passed)
• vp lint on changed TS/TSX files (0 errors)
• vp fmt --check on changed TS/TSX files
• git diff --check

Note: full-project vp check --fix could not complete in the isolated worktree due to unresolved workspace dependency/types outside these files; targeted lint and tests passed.