feat: extract secrets to .env, remove hardcoded credentials

- Backend: .env file for DB/JWT/Redis/MinIO config, appsettings.json cleared
- Backend: Program.cs loads .env at startup (no extra NuGet packages)
- Frontend: .env files for VITE_API_URL, api-clients use import.meta.env
- Added vite-env.d.ts type declarations for both frontends
- All hardcoded localhost:5000 replaced with env variable
- Added .env.example template for onboarding

frontend-doctor/src/assets/styles/variables.css

@@ -0,0 +1,51 @@
+:root {
+  --color-primary: #4F6EF7;
+  --color-primary-hover: #3D56D6;
+  --color-primary-bg: #EDF0FD;
+  --color-primary-gradient: linear-gradient(135deg, #4F6EF7 0%, #6C8AFF 100%);
+
+  --color-danger: #EF4444;
+  --color-danger-bg: #FEE9E9;
+  --color-success: #20C997;
+  --color-success-bg: #E6F9F2;
+  --color-warning: #F59E0B;
+  --color-warning-bg: #FFF8E6;
+  --color-info: #339AF0;
+  --color-info-bg: #EFF6FF;
+  --color-purple: #845EF7;
+  --color-purple-bg: #F3E8FF;
+  --color-pink: #F06595;
+  --color-pink-bg: #FFF0F5;
+
+  --color-white: #FFFFFF;
+  --color-bg: #F5F7FB;
+  --color-bg-secondary: #EDF0F7;
+  --color-text-primary: #1A1D28;
+  --color-text-secondary: #5A6072;
+  --color-text-tertiary: #9BA0B4;
+  --color-text-inverse: #FFFFFF;
+  --color-border: #E1E5ED;
+  --color-divider: #EEF0F5;
+
+  --shadow-sm: 0 1px 3px rgba(0, 0, 0, 0.04);
+  --shadow-md: 0 4px 16px rgba(0, 0, 0, 0.06);
+  --shadow-lg: 0 8px 30px rgba(0, 0, 0, 0.08);
+  --shadow-primary: 0 4px 16px rgba(79, 110, 247, 0.25);
+
+  --radius-sm: 8px;
+  --radius-md: 12px;
+  --radius-lg: 16px;
+  --radius-xl: 20px;
+  --radius-full: 9999px;
+
+  --font-size-xs: 11px;
+  --font-size-sm: 13px;
+  --font-size-base: 14px;
+  --font-size-md: 15px;
+  --font-size-lg: 17px;
+  --font-size-xl: 20px;
+  --font-size-2xl: 24px;
+  --font-size-3xl: 30px;
+
+  --font-family: 'PingFang SC', -apple-system, BlinkMacSystemFont, 'Segoe UI', 'Microsoft YaHei', sans-serif;
+}