Add Prettier to the frontend

Add prettier and prettier-plugin-svelte as dev dependencies (R2), a
.prettierrc matching the current style with a 120-character width, a
.prettierignore for dist, bindings and node_modules, and the format and
format:check npm scripts. The reformat itself follows in its own commit.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/.prettierignore

@@ -0,0 +1,3 @@
+dist
+bindings
+node_modules

frontend/.prettierrc

@@ -0,0 +1,7 @@
+{
+  "printWidth": 120,
+  "semi": true,
+  "singleQuote": false,
+  "trailingComma": "all",
+  "plugins": ["prettier-plugin-svelte"]
+}

frontend/package-lock.json

@@ -15,6 +15,8 @@
       },
       "devDependencies": {
         "@sveltejs/vite-plugin-svelte": "^7.0.0",
+        "prettier": "^3.9.10",
+        "prettier-plugin-svelte": "^4.1.1",
         "svelte": "^5.46.4",
         "svelte-check": "^4.4.8",
         "tslib": "^2.6.2",
@@ -978,6 +980,36 @@
         "node": "^10 || ^12 || >=14"
       }
     },
+    "node_modules/prettier": {
+      "version": "3.9.10",
+      "resolved": "https://registry.npmjs.org/prettier/-/prettier-3.9.10.tgz",
+      "integrity": "sha512-skGHTH1Yb9QvaTC4hWnRTzEYtCpApF0UMBPz+t/0ZPzP8ihefo/btKd+QA7pUE0fEwA1SbS/NmhyEcstdXOIdA==",
+      "dev": true,
+      "license": "MIT",
+      "bin": {
+        "prettier": "bin/prettier.cjs"
+      },
+      "engines": {
+        "node": ">=14"
+      },
+      "funding": {
+        "url": "https://github.com/prettier/prettier?sponsor=1"
+      }
+    },
+    "node_modules/prettier-plugin-svelte": {
+      "version": "4.1.1",
+      "resolved": "https://registry.npmjs.org/prettier-plugin-svelte/-/prettier-plugin-svelte-4.1.1.tgz",
+      "integrity": "sha512-wXvbXMjSvb4C9ENWTHXyd+ihakKCsJ6rJhLP6/8HFNj4GkZr48jqL9PoKsl2sk7SyCZRTnJ7O2TTowUpOxP/KA==",
+      "dev": true,
+      "license": "MIT",
+      "engines": {
+        "node": ">=20"
+      },
+      "peerDependencies": {
+        "prettier": "^3.0.0",
+        "svelte": "^5.0.0"
+      }
+    },
     "node_modules/readdirp": {
       "version": "4.1.2",
       "resolved": "https://registry.npmjs.org/readdirp/-/readdirp-4.1.2.tgz",

frontend/package.json

@@ -8,7 +8,9 @@
     "build:dev": "vite build --minify false --mode development",
     "build": "vite build --mode production",
     "preview": "vite preview",
-    "check": "svelte-check --tsconfig ./tsconfig.json"
+    "check": "svelte-check --tsconfig ./tsconfig.json",
+    "format": "prettier --write .",
+    "format:check": "prettier --check ."
   },
   "dependencies": {
     "@fontsource/ibm-plex-mono": "^5.3.0",
@@ -18,6 +20,8 @@
   },
   "devDependencies": {
     "@sveltejs/vite-plugin-svelte": "^7.0.0",
+    "prettier": "^3.9.10",
+    "prettier-plugin-svelte": "^4.1.1",
     "svelte": "^5.46.4",
     "svelte-check": "^4.4.8",
     "tslib": "^2.6.2",