feat: Phase 1 frontend - auth UI with KITHARA_MODE switch

- Auth API client (src/api/auth.ts): login, register, me, api-key endpoints
- AuthContext: React context managing auth state, mode detection, JWT tokens
- LoginPage: clean dark-themed login form with error handling
- RegisterPage: invite code + registration with password validation
- App.tsx: BrowserRouter + AuthProvider wrapper, mode-conditional routing
  - Single mode: straight to app, no auth required
  - Multi mode: redirect to /login, auth-gated access
- main.tsx: BrowserRouter + AuthProvider initialization
- react-router-dom added as dependency
- docker-compose.yml: KITHARA_MODE, KITHARA_ADMIN_USER, KITHARA_ADMIN_PASS env vars
- requirements.txt: pinned bcrypt==4.0.1 for passlib compatibility
- Tested: multi-mode admin bootstrap, JWT login, auth/me round-trip all confirmed

frontend/package-lock.json

@@ -10,7 +10,8 @@
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         "react": "^18.3.0",
-        "react-dom": "^18.3.0"
+        "react-dom": "^18.3.0",
+        "react-router-dom": "^7.16.0"
       },
       "devDependencies": {
         "@types/react": "^18.3.0",
@@ -1518,6 +1519,19 @@
       "dev": true,
       "license": "MIT"
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+      "license": "MIT",
+      "engines": {
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+      "funding": {
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@@ -2287,6 +2301,44 @@
         "node": ">=0.10.0"
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+    "node_modules/react-router": {
+      "version": "7.16.0",
+      "resolved": "https://registry.npmjs.org/react-router/-/react-router-7.16.0.tgz",
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+      "license": "MIT",
+      "dependencies": {
+        "cookie": "^1.0.1",
+        "set-cookie-parser": "^2.6.0"
+      },
+      "engines": {
+        "node": ">=20.0.0"
+      },
+      "peerDependencies": {
+        "react": ">=18",
+        "react-dom": ">=18"
+      },
+      "peerDependenciesMeta": {
+        "react-dom": {
+          "optional": true
+        }
+      }
+    },
+    "node_modules/react-router-dom": {
+      "version": "7.16.0",
+      "resolved": "https://registry.npmjs.org/react-router-dom/-/react-router-dom-7.16.0.tgz",
+      "integrity": "sha512-kMUAbimWB5FVbF4Bce4bJsiKJWLIUHq/mEG8+CFDnCSgltptBiG5nguducmsJeGKytlCvQud9Qhzpn49iduTlA==",
+      "license": "MIT",
+      "dependencies": {
+        "react-router": "7.16.0"
+      },
+      "engines": {
+        "node": ">=20.0.0"
+      },
+      "peerDependencies": {
+        "react": ">=18",
+        "react-dom": ">=18"
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+    },
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@@ -2431,6 +2483,12 @@
         "semver": "bin/semver.js"
       }
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+    "node_modules/set-cookie-parser": {
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+      "license": "MIT"
+    },
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