refactor: migrate main.py to modular routers and add project roadmap

- Migrated monolithic main.py to feature-scoped routers in app/routers/
- Added GEMINI.md for project context and AI instructional guidelines
- Updated README.md with a comprehensive modernization plan (SQL migration, robust scraping DSL, frontend modernization)
- Improved authentication with cookie support and modular JS
- Updated test suite and documentation

static/js/auth-utils.js

@@ -0,0 +1,105 @@
+/**
+ * Auth utilities - safe JSON parsing and error display
+ * Following the pattern from static/js/watchlist.js (global exports)
+ */
+
+// API base URL - use relative path for same-origin
+const API_BASE = '/api';
+
+/**
+ * Safely parse JSON string with fallback
+ * @param {string} text - The JSON string to parse
+ * @param {*} fallback - The fallback value if parsing fails (default: null)
+ * @returns {*} Parsed object or fallback value
+ */
+function safeJsonParse(text, fallback = null) {
+  try {
+    if (text === undefined || text === null || text === '') {
+      return fallback;
+    }
+    return JSON.parse(text);
+  } catch (error) {
+    console.error('JSON parse error:', error.message);
+    return fallback;
+  }
+}
+
+/**
+ * Display error message in the specified element
+ * Handles string, object, and array errors properly
+ * @param {string} elementId - The ID of the element to display error in
+ * @param {*} error - The error (string, object, or array)
+ * @param {string} defaultMessage - Default message if error is invalid
+ */
+function displayError(elementId, error, defaultMessage = 'Une erreur est survenue') {
+  const errorDiv = document.getElementById(elementId);
+  if (!errorDiv) {
+    console.error('Error element not found:', elementId);
+    return;
+  }
+
+  let message = defaultMessage;
+
+  if (error === null || error === undefined) {
+    message = defaultMessage;
+  } else if (typeof error === 'string') {
+    message = error;
+  } else if (typeof error === 'object') {
+    // Handle array errors
+    if (Array.isArray(error)) {
+      message = error.join('\n');
+    } 
+    // Handle FastAPI HTTPException detail (can be string or object)
+    else if (error.detail) {
+      if (typeof error.detail === 'string') {
+        message = error.detail;
+      } else if (typeof error.detail === 'object' && error.detail.msg) {
+        message = error.detail.msg;
+      } else {
+        // Stringify the object to avoid "[object Object]"
+        message = JSON.stringify(error.detail);
+      }
+    }
+    // Handle generic object
+    else {
+      message = JSON.stringify(error);
+    }
+  }
+
+  errorDiv.textContent = message;
+  errorDiv.classList.add('show');
+  
+  // Hide success message if visible
+  const successDiv = document.getElementById(elementId.replace('Error', 'Success'));
+  if (successDiv) {
+    successDiv.classList.remove('show');
+  }
+}
+
+/**
+ * Display success message in the specified element
+ * @param {string} elementId - The ID of the element to display success in
+ * @param {string} message - The success message
+ */
+function displaySuccess(elementId, message) {
+  const successDiv = document.getElementById(elementId);
+  if (!successDiv) {
+    console.error('Success element not found:', elementId);
+    return;
+  }
+
+  successDiv.textContent = message;
+  successDiv.classList.add('show');
+  
+  // Hide error message if visible
+  const errorDiv = document.getElementById(elementId.replace('Success', 'Error'));
+  if (errorDiv) {
+    errorDiv.classList.remove('show');
+  }
+}
+
+// Export globally (following watchlist.js pattern)
+window.safeJsonParse = safeJsonParse;
+window.displayError = displayError;
+window.displaySuccess = displaySuccess;
+window.API_BASE = API_BASE;