Pasted prompts (test cases, structured asks, JSON dumps) currently render
through the same markdown pipeline as assistant output, so '#'/'-'/'**'
characters are processed and long prompts dominate the scrollback.
- New UserMessageContent.vue: plain-text rendering (white-space: pre-wrap
preserves user-typed newlines and indentation), with auto-collapse beyond
8 lines and a "Show more (N more lines) / Show less" toggle. Soft mask
gradient at the collapse boundary instead of a hard cut.
- MessageBubble.vue: route role==='user' messages through the new component;
assistant messages keep the existing markdown pipeline unchanged.
- Add chat.expandLines / chat.collapse i18n keys (zh + en).
Verified end-to-end in browser preview: 15-line content collapses to 8,
toggle expands to full 15 with "Show less" label, raw '#' / '**' / '`' chars
shown literally with no <strong>/<h1>/<li> tags emitted.
