fix(viewer): preserve semantic response chunks

[Lint111]

Summary

Make the response viewer preserve semantic reply boundaries and support lossless whole-chunk copy on desktop and mobile.

Changes

• Remove terminal-buffer fallback from Last Response.
  • An empty structured transcript now shows the existing empty-state message.
  • It no longer presents the entire terminal history as if it were one agent reply.
• Store each reply's raw transcript text on its owning response node.
• Double-click or double-tap a reply to copy that complete raw chunk.
• Ignore copy gestures that originate from buttons, links, fields, or editable content.
• Keep existing one-click code-block copy and diagram wrap controls unchanged.
• Show brief visual confirmation after a successful whole-chunk copy.
• Use mobile terminal feedback when that optional control layer is present.

Behavior

• Last Response remains a single structured reply.
• More remains the explicit path to the full conversation.
• Raw Markdown, whitespace, and code fences survive whole-reply copy without depending on rendered HTML.

Validation

• npx vitest run --config config/vitest.config.ts test/response-viewer.test.ts
  • 2 tests passed
• npm run check:frontend-syntax
  • 26 frontend files passed
• npm run build
  • passed

Scope

This PR does not add click-to-collapse for response messages. That interaction was discussed but is not implemented in the audited change set and should be reviewed independently from copy/source correctness.