Styled output and collapsible command boxes in web app

[aseemrb]

All changes are for styling only in web/, no Java or core code changes:

• Each command and its output is now a collapsible box. The header shows the command, any TRUE/FALSE results, and the elapsed time, so a collapsed transcript still shows what each command concluded. A "Collapse outputs" / "Expand outputs" button in the top bar toggles all boxes.
• Output is rendered by line type instead of as raw text:
  • eval/def results (____ followed by TRUE or FALSE) are shown as a green or red pill.
  • Evaluation steps from : and :: are shown as a tree: expression on the left, state count and time on the right, indented by depth. The verbose computing/computed lines are muted. Counts of 100,000 states or more are highlighted. Steps are grouped under a fold that is open by default.
  • "Total computation time" is shown as a muted footer.
  • Walnut error messages are shown in the error color; Java stack-trace frames are folded behind a "stack trace" toggle; the internal /walnut/ prefix is stripped from paths.
  • test output is shown as a row of chips with a count.
  • help output gets a heading and readable prose; the === rules are dropped.
  • Commands echoed by load are styled like the prompt lines.

The line patterns match formats that each come from a single place in the core (EvalDef, Logging, the help printer).