Formalize the definition of Project::$categories

Problem/Motivation

The Project object's categories property is very poorly defined. The intention, based on my reading of the Svelte code, is that it's meant to be an array like [$some_id => ['name' => 'Foo'], ...], but that's overcomplicated and weird.

Proposed resolution

Formally define Project::$categories as a simple associative array. The keys are category IDs (int or string), and the values are strings with the human-facing name of that category. We'll need to adjust the Svelte code to match, and likely a couple of sources will need some minor changes for this too.

Since this is an API refinement (although not exactly a "change"), it is a stable blocker.