fix: add Thumbnail field and video result template

MainResult: add Thumbnail field (used by YouTube, images, etc.)
video_item.html: new partial for video results with thumbnail display
views.go: add templateForResult func + video_item.html to template parse
results_inner.html: dispatch to video_item when Template="videos"
kafka.css: add .video-result flex layout with thumbnail styling

internal/contracts/main_result.go

@@ -14,16 +14,17 @@ type MainResult struct {
 	raw map[string]any
 
 	// Common fields used by templates (RSS uses: title, url, content, pubdate).
-	Template string `json:"template"`
-	Title    string `json:"title"`
-	Content  string `json:"content"`
-	URL      *string `json:"url"`
-	Pubdate  *string `json:"pubdate"`
+	Template  string `json:"template"`
+	Title     string `json:"title"`
+	Content   string `json:"content"`
+	URL       *string `json:"url"`
+	Pubdate   *string `json:"pubdate"`
+	Thumbnail string  `json:"thumbnail"`
 
-	Engine  string  `json:"engine"`
-	Score   float64 `json:"score"`
-	Category string `json:"category"`
-	Priority string `json:"priority"`
+	Engine   string   `json:"engine"`
+	Score    float64  `json:"score"`
+	Category string   `json:"category"`
+	Priority string   `json:"priority"`
 
 	Positions []int    `json:"positions"`
 	Engines   []string `json:"engines"`
@@ -54,6 +55,7 @@ func (mr *MainResult) UnmarshalJSON(data []byte) error {
 	mr.Title = stringOrEmpty(m["title"])
 	mr.Content = stringOrEmpty(m["content"])
 	mr.Engine = stringOrEmpty(m["engine"])
+	mr.Thumbnail = stringOrEmpty(m["thumbnail"])
 	mr.Category = stringOrEmpty(m["category"])
 	mr.Priority = stringOrEmpty(m["priority"])
 
@@ -93,20 +95,21 @@ func (mr MainResult) MarshalJSON() ([]byte, error) {
 
 	// Otherwise, marshal the known fields.
 	m := map[string]any{
-		"template": mr.Template,
-		"title":    mr.Title,
-		"content":  mr.Content,
-		"url":      mr.URL,
-		"pubdate":  mr.Pubdate,
-		"engine":   mr.Engine,
-		"score":    mr.Score,
-		"category": mr.Category,
-		"priority": mr.Priority,
+		"template":  mr.Template,
+		"title":     mr.Title,
+		"content":   mr.Content,
+		"url":       mr.URL,
+		"pubdate":   mr.Pubdate,
+		"thumbnail": mr.Thumbnail,
+		"engine":    mr.Engine,
+		"score":     mr.Score,
+		"category":  mr.Category,
+		"priority":  mr.Priority,
 		"positions": mr.Positions,
 		"engines":   mr.Engines,
-		"open_group": mr.OpenGroup,
+		"open_group":  mr.OpenGroup,
 		"close_group": mr.CloseGroup,
-		"parsed_url": mr.ParsedURL,
+		"parsed_url":  mr.ParsedURL,
 	}
 	return json.Marshal(m)
 }