chore(deps): Node 22 → 26 for the docs and notebooks builds

Node 26 is the current LTS. It ships without corepack (dropped from
Node 25 on), so both build stages died at `corepack enable` ("not
found"). Each now installs a pinned corepack from npm first
(COREPACK_VERSION=0.36.0, renovate-annotated so it gets bumped too).

Verified: both images build on node 26; the notebooks image passes
nb_fe_smoke --image (marimo home and editor, no console errors); the
docs site renders /, /api/ and search with no page errors.

infra/images/docs.Dockerfile

@@ -19,12 +19,15 @@ RUN uv run --with griffe python docs/scripts/extract_docs.py
 
 
 # Stage 2: Build Docusaurus static site
-FROM node:22-slim AS build
+FROM node:26-slim AS build
 
 # No `corepack prepare <version>`: corepack runs the pnpm that
 # docs/package.json's packageManager names, so Renovate's bump of that
 # field is the whole upgrade (a second pin here went stale at pnpm 12).
-RUN corepack enable
+# Node 25+ no longer bundles corepack, so install a pinned one from npm.
+# renovate: datasource=npm depName=corepack
+ARG COREPACK_VERSION=0.36.0
+RUN npm install -g corepack@${COREPACK_VERSION} && corepack enable
 
 WORKDIR /docs
 

infra/images/notebooks.Dockerfile

@@ -22,13 +22,17 @@ ARG MARIMO_VERSION=0.25.2
 
 # ---- stage 1: frontend ------------------------------------------------------
 
-FROM node:22-bookworm-slim AS fe
+FROM node:26-bookworm-slim AS fe
 ARG MARIMO_VERSION
 WORKDIR /src
 
+# Node 25+ no longer bundles corepack, so install a pinned one from npm.
+# renovate: datasource=npm depName=corepack
+ARG COREPACK_VERSION=0.36.0
 RUN apt-get update \
     && apt-get install -y --no-install-recommends git ca-certificates python3 jq \
     && rm -rf /var/lib/apt/lists/* \
+    && npm install -g corepack@${COREPACK_VERSION} \
     && corepack enable
 # No `corepack prepare pnpm@latest`: the corepack shim resolves the exact
 # pnpm version from marimo's package.json `packageManager` field at first