Fix typos in comments, docs and spec descriptions #5741

There are about 20 spelling mistakes in code comments, docs (user guide, ADRs, wiki) and spec descriptions. Examples:

• "occurrance" in deadline_day_controller.js
• "attemping" in four system spec names
• "accidently" in the partners user guide
• "Navigaton" in an inventory-audits image alt text

I'd like to fix them in one small PR. It would change no behavior: no identifiers, routes, i18n keys, data or fixtures.

A PR is coming shortly. It was prepared with AI assistance (Claude Code), and that is disclosed in the PR.