feat: implement universal icon system with Phosphor Icons

- Install @phosphor-icons/react package
- Create centralized Icon component with size/weight/color variants
- Create icon registry with 34 icons across 5 categories
- Replace all raw Unicode symbols with proper icon components
- Add icons to navigation, buttons, status indicators, git operations
- Add icon CSS with consistent sizing and spacing
- Fix type definitions for Phosphor icon compatibility

Quality gates: typecheck ✓, lint ✓, build ✓ (375KB bundle)

apps/web/package-lock.json

@@ -8,6 +8,7 @@
       "name": "headquarter-web",
       "version": "0.1.0",
       "dependencies": {
+        "@phosphor-icons/react": "^2.1.10",
         "@types/prismjs": "^1.26.6",
         "axios": "^1.6.0",
         "prismjs": "^1.30.0",
@@ -1271,6 +1272,19 @@
         "url": "https://github.com/sponsors/Boshen"
       }
     },
+    "node_modules/@phosphor-icons/react": {
+      "version": "2.1.10",
+      "resolved": "https://registry.npmjs.org/@phosphor-icons/react/-/react-2.1.10.tgz",
+      "integrity": "sha512-vt8Tvq8GLjheAZZYa+YG/pW7HDbov8El/MANW8pOAz4eGxrwhnbfrQZq0Cp4q8zBEu8NIhHdnr+r8thnfRSNYA==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=10"
+      },
+      "peerDependencies": {
+        "react": ">= 16.8",
+        "react-dom": ">= 16.8"
+      }
+    },
     "node_modules/@remix-run/router": {
       "version": "1.23.2",
       "resolved": "https://registry.npmjs.org/@remix-run/router/-/router-1.23.2.tgz",