feat: add platform configuration tables and admin settings

Add four DB-backed configuration tables for site-wide settings:
site_settings (logo, top-left title, header/footer text with zh/en
variants), upload_configs (master switch, default size, storage dir),
upload_file_types (per-extension whitelist with per-type size limits),
and download_baseurls (multiple download sources with priority/default).

- Models, AutoMigrate, and first-run seed (18 common file types)
- Process-level config cache warmed at startup, refreshed on admin save
- SetUserContext injects cached site settings into templates
- base.html renders logo (local upload or external URL), title, header
  banner, and footer with i18n fallback
- Upload validator drives avatar uploads (form + AJAX) through the
  configured switch/type/size policy
- Admin pages for site/upload/download settings with i18n keys

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

handlers/profile.go

@@ -46,6 +46,14 @@ func ProfilePage(db *gorm.DB) gin.HandlerFunc {
 		if msg := c.Query("error"); msg == "pw" {
 			data["Error"] = tr["profile_wrong_password"]
 		}
+		switch c.Query("error") {
+		case "upload":
+			data["Error"] = tr["profile_upload_invalid"]
+		case "upload_disabled":
+			data["Error"] = tr["profile_upload_disabled"]
+		case "size":
+			data["Error"] = fmt.Sprintf(tr["profile_upload_too_large"], c.Query("max"))
+		}
 
 		c.HTML(http.StatusOK, "profile", data)
 	}
@@ -84,12 +92,23 @@ func UpdateProfile(db *gorm.DB, storagePath string) gin.HandlerFunc {
 		if err == nil {
 			defer file.Close()
 
-			// Determine file extension.
-			ext := strings.ToLower(filepath.Ext(header.Filename))
-			if ext == "" || (ext != ".jpg" && ext != ".jpeg" && ext != ".png" && ext != ".gif" && ext != ".webp") {
-				ext = ".jpg"
+			// Validate against the platform upload policy (switch + type + size).
+			check := ValidateUpload(header)
+			if !check.OK {
+				session.Save()
+				reason := "?error=upload"
+				if !models.GetUploadConfig().Enabled {
+					reason = "?error=upload_disabled"
+				} else if check.Type != nil {
+					reason = fmt.Sprintf("?error=size&max=%s", formatSize(check.MaxSize))
+				}
+				c.Redirect(http.StatusFound, "/profile"+reason)
+				return
 			}
 
+			// Determine file extension (validated to be in the whitelist).
+			ext := strings.ToLower(filepath.Ext(header.Filename))
+
 			// Save under storagePath/avatars/.
 			avatarDir := filepath.Join(storagePath, "avatars")
 			if err := os.MkdirAll(avatarDir, 0755); err != nil {
@@ -173,12 +192,24 @@ func UploadAvatar(db *gorm.DB, storagePath string) gin.HandlerFunc {
 		}
 		defer file.Close()
 
-		// Validate extension.
-		ext := strings.ToLower(filepath.Ext(header.Filename))
-		if ext == "" || (ext != ".jpg" && ext != ".jpeg" && ext != ".png" && ext != ".gif" && ext != ".webp") {
-			ext = ".jpg"
+		// Validate against the platform upload policy (switch + type + size).
+		check := ValidateUpload(header)
+		if !check.OK {
+			if !models.GetUploadConfig().Enabled {
+				c.JSON(http.StatusBadRequest, gin.H{"error": "uploads are disabled"})
+				return
+			}
+			if check.Type != nil {
+				c.JSON(http.StatusBadRequest, gin.H{"error": fmt.Sprintf("file too large; limit is %s", formatSize(check.MaxSize))})
+				return
+			}
+			c.JSON(http.StatusBadRequest, gin.H{"error": "file type not allowed"})
+			return
 		}
 
+		// Determine file extension (validated to be in the whitelist).
+		ext := strings.ToLower(filepath.Ext(header.Filename))
+
 		// Read file bytes for image processing.
 		imgBytes, err := io.ReadAll(file)
 		if err != nil {