refactor(web): drop TypeScript for plain JavaScript

Every .ts becomes .js, every lang="ts" becomes lang-less, and the type
declarations go with them: types.ts held nothing but types, so it is
deleted outright.

Tooling follows. typescript, svelte-check, typescript-eslint and
@types/sql.js are uninstalled; tsconfig.json becomes jsconfig.json, which
still extends the generated SvelteKit config so $lib and $app resolve in
an editor; `npm run lint` is now ESLint alone, and CI's comment about it
covering the type check goes too.

What this gives up, stated plainly: a mistyped column name like
row.nguvan used to fail the build and now renders blank, and the
datasets.json-to-CONTENT cross-check is back to throwing at module load
rather than at compile time. The runtime guard for the latter is still
there and still throws loudly.

Two mechanical notes. The svelte/no-navigation-without-resolve rule
started flagging the footer's source link, which points at an off-site
article — without type information the rule can no longer tell an
external URL from a route, so that one line carries a disable comment.
And Vitest's include pattern had to follow the tests to .js.

Lint, 25 tests and the build all pass.

.github/workflows/deploy-pages.yml

@@ -66,8 +66,8 @@ jobs:
       - name: Test assembler
         run: go -C assembler test ./...
 
-      # Lint covers ESLint and svelte-check; the tests cover the framework-free
-      # modules, including the ASCII fold that has to match the Go parser.
+      # The tests cover the framework-free modules, including the ASCII fold
+      # that has to match the Go parser.
       - name: Test web
         working-directory: web
         run: npm test