Add Prettier (config + scripts), not yet applied

printWidth 100 to stay close to the existing hand-formatting; single
quotes; trailing commas. `npm run format` / `npm run format:check`.
The formatting pass itself is the next commit so it stays isolated.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017Ve7HLspzeG2xDPtJQ8vmu

.prettierignore

@@ -0,0 +1,7 @@
+dist
+dev-dist
+node_modules
+package-lock.json
+src/data/snapshot.json
+docs/screenshots
+CHANGELOG.md

.prettierrc.json

@@ -0,0 +1,5 @@
+{
+  "singleQuote": true,
+  "printWidth": 100,
+  "trailingComma": "all"
+}

package-lock.json

@@ -17,6 +17,7 @@
         "workbox-window": "^7.4.1"
       },
       "devDependencies": {
+        "prettier": "3.9.6",
         "vite": "^6.4.3",
         "vite-plugin-pwa": "^1.3.0"
       },
@@ -4925,6 +4926,22 @@
         "node": "^10 || ^12 || >=14"
       }
     },
+    "node_modules/prettier": {
+      "version": "3.9.6",
+      "resolved": "https://registry.npmjs.org/prettier/-/prettier-3.9.6.tgz",
+      "integrity": "sha512-OpN0zzVdiaiAhxpuuj5efpIS4sY9j7bY6uR5mnj5yPzGkdkjNKSJeUThPb60Jw29QuAZgA4o+/iB49kFiaBX6g==",
+      "dev": true,
+      "license": "MIT",
+      "bin": {
+        "prettier": "bin/prettier.cjs"
+      },
+      "engines": {
+        "node": ">=14"
+      },
+      "funding": {
+        "url": "https://github.com/prettier/prettier?sponsor=1"
+      }
+    },
     "node_modules/pretty-bytes": {
       "version": "6.1.1",
       "resolved": "https://registry.npmjs.org/pretty-bytes/-/pretty-bytes-6.1.1.tgz",

package.json

@@ -9,13 +9,16 @@
     "prebuild": "node scripts/build-snapshot.mjs",
     "dev": "vite",
     "build": "vite build",
-    "preview": "vite preview"
+    "preview": "vite preview",
+    "format": "prettier --write .",
+    "format:check": "prettier --check ."
   },
   "engines": {
     "node": ">=20"
   },
   "license": "MIT",
   "devDependencies": {
+    "prettier": "3.9.6",
     "vite": "^6.4.3",
     "vite-plugin-pwa": "^1.3.0"
   },