chore(admin-web): add Prettier formatter with Svelte support

Add prettier + prettier-plugin-svelte as devDependencies with a
.prettierrc.json matching the existing code style (tabs, single quotes,
semicolons, trailing commas, 120 char width). Add .prettierignore for
build artifacts and lockfile. Wire up `npm run format` (write) and
`npm run format:check` (CI gate) scripts.

Prettier is chosen over Biome because its prettier-plugin-svelte correctly
preserves <script> block indentation per Svelte convention; Biome's
experimental Svelte formatter flattens script-block indentation to column 0,
producing inconsistent output.

hub/admin-web/.prettierrc.json

@@ -0,0 +1,9 @@
+{
+	"useTabs": true,
+	"singleQuote": true,
+	"semi": true,
+	"trailingComma": "all",
+	"printWidth": 120,
+	"plugins": ["prettier-plugin-svelte"],
+	"overrides": [{ "files": "*.svelte", "options": { "parser": "svelte" } }]
+}