enhance: detect duplicate input ids in lab guide questions

[italovalcy]

What

Extends parseLabGuide() in apps/templates/pages/labs_edit.html to also reject the lab-guide form when two inputs share the same id, in addition to the existing duplicate-name check.

Why

radio/checkbox options legitimately repeat their name (they form a single option group), so the name check intentionally allows that. But each option must still have a unique id — the Add checkbox question button emits <input id="idN-1"> <label for="idN-1"> pairs, and a duplicate id silently breaks the label association so only the first matching option responds to a label click. Duplicate ids can arise e.g. when a question block is copied or two groups end up with the same id prefix.

Change

An independent id-uniqueness pass runs in the same preview loop (before the name logic, so it also covers inputs missing a name):

• tracks seen ids; a repeated id is collected once (deduplicated) and the input is highlighted red;
• the form aborts if there is a duplicate name or a duplicate id, showing both messages:
  • Duplicated question name found: ...
  • Duplicated input id found: ...

questionCounter is unchanged (still counts unique names).

Tests

tests/js/parseLabGuide.test.js — the per-case expectation is now { name, id } so a case can assert exactly which message fires. Added cases:

• duplicate id within a radio group, ids reused across two groups, duplicate id in a checkbox group, duplicate id on distinct-name text inputs, and a combined name+id case;
• legitimate groups with unique ids (within one group and across two groups) that must not be flagged.

The function is extracted from the template at test time (not copied), so it guards the shipped code. Confirmed the new id cases fail against the pre-change template; 25/25 pass after the change.

cd tests/js && npm install && npm test

Reviewer notes

• Builds on the duplicate-name fix from fix: detect duplicate question names involving select boxes #273 (already merged to main); this branch is cut from current main and contains only the id-detection commit.
• No Python/route/model changes.