chore: pin pnpm via packageManager field for CI and tooling

Adds a packageManager field (pnpm@9.15.9) to package.json so corepack
and pnpm/action-setup@v4 resolve a single pinned pnpm 9 version that
matches lockfileVersion 9.0. Removes the floating 'version: latest' from
the CI and screenshots workflows so they honor the packageManager field
instead of pulling pnpm 10+, which breaks native module builds.

Follow-up to #63.

.github/workflows/ci.yml

@@ -19,8 +19,6 @@ jobs:
       - uses: actions/checkout@v4
 
       - uses: pnpm/action-setup@v4
-        with:
-          version: latest
 
       - uses: actions/setup-node@v4
         with:
@@ -37,8 +35,6 @@ jobs:
       - uses: actions/checkout@v4
 
       - uses: pnpm/action-setup@v4
-        with:
-          version: latest
 
       - uses: actions/setup-node@v4
         with:
@@ -68,8 +64,6 @@ jobs:
       - uses: actions/checkout@v4
 
       - uses: pnpm/action-setup@v4
-        with:
-          version: latest
 
       - uses: actions/setup-node@v4
         with:

.github/workflows/screenshots.yml

@@ -14,8 +14,6 @@ jobs:
       - uses: actions/checkout@v4
 
       - uses: pnpm/action-setup@v4
-        with:
-          version: latest
 
       - uses: actions/setup-node@v4
         with:

package.json

@@ -3,6 +3,7 @@
   "private": true,
   "version": "0.28.3",
   "type": "module",
+  "packageManager": "pnpm@9.15.9",
   "license": "MIT",
   "author": "Bretzel (https://bretzel.app)",
   "homepage": "https://crumbs.bretzel.app",