feat: #30 generate client types from the GraphQL schema

[erikhfj]

Closes #30.

What was done

client/src/types.ts no longer exists (it was removed before M3), so the issue text was stale. The real problem was the same one it described: every result type was hand-written next to its operation, with nothing tying it to schema.graphql. That is how the GenreFacet.genre shape was wrong in M2.

• client/codegen.ts runs the GraphQL Code Generator client preset against the repository's root schema.graphql - the same file the server reads at startup, so there is no second copy of the contract.
• Each operation keeps its place in client/src/api/*.ts but is declared with graphql(). Result types are derived (SearchAlbumsQuery['searchAlbums'], NonNullable<AlbumDetailQuery['album']>, ...), and the component-facing names (AlbumSummary, GenreFacet, AlbumDetail) survive as aliases, so this PR does not touch components - deliberately, to keep it mergeable with Improve UI to match the original design specification #70.
• DateTime is mapped to string; without it the generated type is unknown and the review timestamps stop typechecking.
• Generated output is committed under client/src/generated/, and CI regenerates and fails on any diff before typecheck runs, so the message is "run pnpm codegen" rather than a type error inside generated code.
• Test files are excluded from the document glob. Without that, CrateButton.test.tsx's fixture operation lands in the generated document map and in the production bundle.
• ALBUM_SORTS stays a runtime array (the URL parser iterates it; the generated enum is a type union with nothing to iterate), and a new test compares it to the schema enum so a value cannot go missing.
• pnpm build runs the generator first, which is what makes the acceptance criterion literally true.

What was tested

The acceptance criterion, run for real. Renaming averageRating to averageScore in schema.graphql:

$ pnpm build
client build: Error 0: Cannot query field "averageRating" on type "Album". Did you mean "averageScore"?
ERR_PNPM_RECURSIVE_RUN_FIRST_FAIL @t19/client@0.0.0 build
exit 1

Restored, pnpm build succeeds: index-*.js 418.29 kB (gzip 126.37 kB), inside the 512 000-byte budget.

pnpm format:check && pnpm lint && pnpm typecheck && pnpm test passes: 108 client tests (one new), 61 server tests. pnpm codegen immediately after generation produces no diff, which is what the CI step checks.

Notes for the reviewer

• Two new dev dependencies in client/: @graphql-codegen/cli, @graphql-codegen/client-preset, plus @graphql-typed-document-node/core which the preset's output imports and pnpm does not hoist.
• Generated files are ignored by ESLint and Prettier. Regenerating them into Prettier's style would make every stale-file diff a formatting diff.
• The client preset's document map is not tree-shakeable by design; the measured cost is inside the existing budget, but the bundle number above is the evidence rather than an assumption.

[rachelks]

Looks good! Tested codegen, build and tests locally, all passing. No issues found in the review. Approved, and we can merge after #72 as planned!