chore(frontend): introduce prettier + eslint and prune unused code

- Add Prettier 3 with prettier-plugin-organize-imports (sorts/removes unused imports)
- Add ESLint 10 flat config with typescript-eslint + eslint-plugin-vue + eslint-config-prettier
- Add root scripts: format, format:check, lint, lint:fix
- Reformat 257 files across admin-web, ops-web, desktop-client, packages
- Remove unused locals/exports flagged by --noUnusedLocals/--noUnusedParameters
- Fix duplicate localTabLabel key, surrogate-pair regex u-flag, NBSP literal in regex
- Skip server/ (Go) and apps/browser-extension/ (deprecated per ADR)

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

.prettierrc.json

@@ -0,0 +1,21 @@
+{
+  "semi": false,
+  "singleQuote": true,
+  "trailingComma": "all",
+  "printWidth": 100,
+  "tabWidth": 2,
+  "useTabs": false,
+  "arrowParens": "always",
+  "endOfLine": "lf",
+  "vueIndentScriptAndStyle": false,
+  "htmlWhitespaceSensitivity": "ignore",
+  "plugins": ["prettier-plugin-organize-imports"],
+  "overrides": [
+    {
+      "files": "*.md",
+      "options": {
+        "proseWrap": "preserve"
+      }
+    }
+  ]
+}