Require Node.js >= 25 (engines, installers, Dockerfile, snap, CI, docs) (#7749)

* docs: bump documented Node.js minimum to 25

Etherpad is moving its supported Node.js floor to >= 25 (CI matrix is
already pinned to 25 across all workflows on the node25-corepack-pnpm11
work). Sync the user-facing documentation so the install instructions,
requirements section, and plugin metadata example all reflect the new
minimum instead of Node 22 / 12.17.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* chore: require Node.js >= 25 (engines, installers, Dockerfile, snap, CI)

#7747 added Node 25 *support* but left the floor at Node 22. This
commit completes the cutover so the runtime requirement matches the
documentation bumped in the previous commit.

- package.json: engines.node ">=22.13.0" → ">=25.0.0"
- bin/functions.sh, bin/installer.sh, bin/installer.ps1: REQUIRED_NODE
  bumped to 25 (controls the error message users see when they invoke
  the installer or pnpm scripts on an older Node)
- Dockerfile: base image node:22-alpine → node:25-alpine (×2). Corepack
  comment updated: Node 25 no longer ships corepack at all, so we
  install it from npm rather than refreshing a stale signing-key list
- snap/snapcraft.yaml: pinned NODE_VERSION 22.22.2 → 25.9.0 and the
  surrounding design notes rewritten to reflect Node 25 instead of 22
- .github/workflows/*.yml: matrix dropped from [22, 24, 25] to just
  [25] (anything older now fails engines anyway). Stale comments in
  build-and-deploy-docs.yml referencing vite 8's 22.12 floor cleaned up
- bin/plugins/lib/npmpublish.yml: setup-node 22 → 25 so the plugin
  template propagated to every ether/* plugin matches the new minimum

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* fix(docker): install pnpm directly on Node 25 (no corepack)

node:25-alpine doesn't ship corepack but does pre-install yarn at
/usr/local/bin/yarn, so `npm install -g corepack@latest` fails with
EEXIST trying to register its yarn shim. Per #7747, end-users install
pnpm via plain `npm install -g pnpm` on Node 25 — use the same flow in
the Dockerfile (and remove the unused yarn binary so it doesn't sit on
PATH inside the image). Drops COREPACK_HOME and the related
issue-7687 cache-sharing tweak since there's no corepack shim to share.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

package.json

@@ -42,7 +42,7 @@
     "ui": "link:ui"
   },
   "engines": {
-    "node": ">=22.13.0",
+    "node": ">=25.0.0",
     "pnpm": ">=11.1.2"
   },
   "packageManager": "pnpm@11.1.2",