Add English language support with i18n implementation

Implemented comprehensive internationalization (i18n) for both frontend and backend:

Frontend:
- Added react-i18next configuration with Spanish (default) and English
- Created translation files (locales/es.json, locales/en.json)
- Refactored core components to use i18n: LoginPage, DashboardHeader, DataUploader
- Created LanguageSelector component with toggle between ES/EN
- Updated API client to send Accept-Language header

Backend:
- Created i18n module with translation dictionary for error messages
- Updated security.py to return localized authentication errors
- Updated api/analysis.py to return localized validation errors
- Implemented language detection from Accept-Language header

Spanish remains the default language ensuring backward compatibility.
Users can switch between languages using the language selector in the dashboard header.

https://claude.ai/code/session_1N9VX

frontend/package-lock.json

@@ -12,11 +12,13 @@
         "@radix-ui/react-tooltip": "^1.2.8",
         "clsx": "^2.1.1",
         "framer-motion": "^12.23.24",
+        "i18next": "^25.8.4",
         "lucide-react": "^0.554.0",
         "react": "^19.2.0",
         "react-countup": "^6.5.3",
         "react-dom": "^19.2.0",
         "react-hot-toast": "^2.6.0",
+        "react-i18next": "^16.5.4",
         "recharts": "^3.4.1",
         "xlsx": "^0.18.5"
       },
@@ -261,6 +263,15 @@
         "@babel/core": "^7.0.0-0"
       }
     },
+    "node_modules/@babel/runtime": {
+      "version": "7.28.6",
+      "resolved": "https://registry.npmjs.org/@babel/runtime/-/runtime-7.28.6.tgz",
+      "integrity": "sha512-05WQkdpL9COIMz4LjTxGpPNCdlpyimKppYNoJ5Di5EUObifl8t4tuLuUBBZEpoLYOmfvIWrsp9fCl0HoPRVTdA==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=6.9.0"
+      }
+    },
     "node_modules/@babel/template": {
       "version": "7.27.2",
       "resolved": "https://registry.npmjs.org/@babel/template/-/template-7.27.2.tgz",
@@ -2300,6 +2311,46 @@
         "csstype": "^3.0.10"
       }
     },
+    "node_modules/html-parse-stringify": {
+      "version": "3.0.1",
+      "resolved": "https://registry.npmjs.org/html-parse-stringify/-/html-parse-stringify-3.0.1.tgz",
+      "integrity": "sha512-KknJ50kTInJ7qIScF3jeaFRpMpE8/lfiTdzf/twXyPBLAGrLRTmkz3AdTnKeh40X8k9L2fdYwEp/42WGXIRGcg==",
+      "license": "MIT",
+      "dependencies": {
+        "void-elements": "3.1.0"
+      }
+    },
+    "node_modules/i18next": {
+      "version": "25.8.4",
+      "resolved": "https://registry.npmjs.org/i18next/-/i18next-25.8.4.tgz",
+      "integrity": "sha512-a9A0MnUjKvzjEN/26ZY1okpra9kA8MEwzYEz1BNm+IyxUKPRH6ihf0p7vj8YvULwZHKHl3zkJ6KOt4hewxBecQ==",
+      "funding": [
+        {
+          "type": "individual",
+          "url": "https://locize.com"
+        },
+        {
+          "type": "individual",
+          "url": "https://locize.com/i18next.html"
+        },
+        {
+          "type": "individual",
+          "url": "https://www.i18next.com/how-to/faq#i18next-is-awesome.-how-can-i-support-the-project"
+        }
+      ],
+      "license": "MIT",
+      "dependencies": {
+        "@babel/runtime": "^7.28.4"
+      },
+      "peerDependencies": {
+        "typescript": "^5"
+      },
+      "peerDependenciesMeta": {
+        "typescript": {
+          "optional": true
+        }
+      }
+    },
     "node_modules/immer": {
       "version": "10.2.0",
       "resolved": "https://registry.npmjs.org/immer/-/immer-10.2.0.tgz",
@@ -2518,6 +2569,33 @@
         "react-dom": ">=16"
       }
     },
+    "node_modules/react-i18next": {
+      "version": "16.5.4",
+      "resolved": "https://registry.npmjs.org/react-i18next/-/react-i18next-16.5.4.tgz",
+      "integrity": "sha512-6yj+dcfMncEC21QPhOTsW8mOSO+pzFmT6uvU7XXdvM/Cp38zJkmTeMeKmTrmCMD5ToT79FmiE/mRWiYWcJYW4g==",
+      "license": "MIT",
+      "dependencies": {
+        "@babel/runtime": "^7.28.4",
+        "html-parse-stringify": "^3.0.1",
+        "use-sync-external-store": "^1.6.0"
+      },
+      "peerDependencies": {
+        "i18next": ">= 25.6.2",
+        "react": ">= 16.8.0",
+        "typescript": "^5"
+      },
+      "peerDependenciesMeta": {
+        "react-dom": {
+          "optional": true
+        },
+        "react-native": {
+          "optional": true
+        },
+        "typescript": {
+          "optional": true
+        }
+      }
+    },
     "node_modules/react-is": {
       "version": "19.2.0",
       "resolved": "https://registry.npmjs.org/react-is/-/react-is-19.2.0.tgz",
@@ -2791,7 +2869,7 @@
       "version": "5.8.3",
       "resolved": "https://registry.npmjs.org/typescript/-/typescript-5.8.3.tgz",
       "integrity": "sha512-p1diW6TqL9L07nNxvRMM7hMMw4c5XOo/1ibL4aAIGmSAt9slTE1Xgw5KWuof2uTOvCg9BY7ZRi+GaF+7sfgPeQ==",
-      "dev": true,
+      "devOptional": true,
       "license": "Apache-2.0",
       "bin": {
         "tsc": "bin/tsc",
@@ -2988,6 +3066,15 @@
         }
       }
     },
+    "node_modules/void-elements": {
+      "version": "3.1.0",
+      "resolved": "https://registry.npmjs.org/void-elements/-/void-elements-3.1.0.tgz",
+      "integrity": "sha512-Dhxzh5HZuiHQhbvTW9AMetFfBHDMYpo23Uo9btPXgdYP+3T5S+p+jgNy7spra+veYhBP2dCSgxR/i2Y02h5/6w==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=0.10.0"
+      }
+    },
     "node_modules/wmf": {
       "version": "1.0.2",
       "resolved": "https://registry.npmjs.org/wmf/-/wmf-1.0.2.tgz",