feat: a search box for a matter, in the documents view

Adds the search box the earlier stages were building toward. In a project's
documents view you can now search the whole matter two ways: "Find passages"
lists the best-matching passages with their document, page, and how each was
found (exact words, meaning, an approximate match, or the file's own name);
"Ask the matter" returns one consolidated answer that cites the document and
page, drawn only from the matter's documents. Both call the endpoints added
earlier (GET /search and POST /search/answer); this is the interface for them.

Type-checked against the frontend; contains no build-tool-specific code.