ci: generate docs content before web checks; bun prepare hook for local dev

The first build of #3860 failed at type-check because the generated
`lib/docs-content.generated.ts` is gitignored (regenerated on every
build) and CI's `type-check` step runs before `build`. Two fixes:

- web-ci.yml: explicit `Generate docs content from repo-root docs/`
  step right after `bun install` so format-check, lint, and type-check
  all see the file.
- web/package.json: add `prepare` lifecycle script. `bun install`
  invokes it automatically, so a fresh local checkout boots into a
  working state too.

Build still re-runs the generator via prebuild, so docs/ edits land in
the bundle without an explicit dev action.

.github/workflows/web-ci.yml

@@ -34,6 +34,9 @@ jobs:
       - name: Install dependencies
         run: bun install --frozen-lockfile
 
+      - name: Generate docs content from repo-root docs/
+        run: node ./scripts/generate-docs-content.mjs
+
       - name: Format check
         run: bun run format:check