feat: add frontend logger module with level filtering and sanitization (refs #84)

- Create centralized logger utility at frontend/src/utils/logger.ts
- Support debug/info/warn/error levels controlled by VITE_LOG_LEVEL
- Sanitize sensitive data (tokens, passwords, secrets) in log output
- Graceful fallback to 'info' level for invalid VITE_LOG_LEVEL values
- Add VITE_LOG_LEVEL to ImportMetaEnv type definitions

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/utils/logger.ts

@@ -0,0 +1,94 @@
+/**
+ * @ai-summary Centralized frontend logger with level filtering and sanitization
+ * @ai-context Use this instead of console.log for consistent logging
+ */
+
+type LogLevel = 'debug' | 'info' | 'warn' | 'error';
+
+const LEVELS: Record<LogLevel, number> = {
+  debug: 0,
+  info: 1,
+  warn: 2,
+  error: 3,
+};
+
+const validLevels: LogLevel[] = ['debug', 'info', 'warn', 'error'];
+
+const rawLevel = (
+  import.meta.env.VITE_LOG_LEVEL || 'info'
+).toLowerCase() as LogLevel;
+const LOG_LEVEL: LogLevel = validLevels.includes(rawLevel) ? rawLevel : 'info';
+
+// Warn once on startup if invalid level
+if (import.meta.env.VITE_LOG_LEVEL && rawLevel !== LOG_LEVEL) {
+  console.warn(
+    `[Logger] Invalid VITE_LOG_LEVEL "${import.meta.env.VITE_LOG_LEVEL}", using "info"`
+  );
+}
+
+const shouldLog = (level: LogLevel): boolean => LEVELS[level] >= LEVELS[LOG_LEVEL];
+
+// Sanitize sensitive data from logs
+const sensitiveKeys = [
+  'token',
+  'password',
+  'secret',
+  'authorization',
+  'apikey',
+  'credential',
+];
+
+const sanitize = (meta?: object): object | undefined => {
+  if (!meta) return undefined;
+  const sanitized = { ...meta };
+  for (const key of Object.keys(sanitized)) {
+    if (sensitiveKeys.some((s) => key.toLowerCase().includes(s))) {
+      (sanitized as Record<string, unknown>)[key] = '[REDACTED]';
+    }
+  }
+  return sanitized;
+};
+
+export const logger = {
+  debug: (msg: string, meta?: object): void => {
+    if (shouldLog('debug')) {
+      try {
+        console.debug(`[DEBUG] ${msg}`, sanitize(meta) ?? '');
+      } catch {
+        /* Fail silently */
+      }
+    }
+  },
+
+  info: (msg: string, meta?: object): void => {
+    if (shouldLog('info')) {
+      try {
+        console.log(`[INFO] ${msg}`, sanitize(meta) ?? '');
+      } catch {
+        /* Fail silently */
+      }
+    }
+  },
+
+  warn: (msg: string, meta?: object): void => {
+    if (shouldLog('warn')) {
+      try {
+        console.warn(`[WARN] ${msg}`, sanitize(meta) ?? '');
+      } catch {
+        /* Fail silently */
+      }
+    }
+  },
+
+  error: (msg: string, meta?: object): void => {
+    if (shouldLog('error')) {
+      try {
+        console.error(`[ERROR] ${msg}`, sanitize(meta) ?? '');
+      } catch {
+        /* Fail silently */
+      }
+    }
+  },
+};
+
+export default logger;