feat: add server-side theme cookie with dropdown selector (no JS)

- Add theme POST handler that sets HttpOnly cookie
- Update preferences page to use <select> dropdown instead of JS buttons
- Theme cookie set on POST /preferences with theme parameter
- Theme read from cookie on all page renders
- No JavaScript required for theme selection

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/views/views.go

@@ -55,6 +55,8 @@ type PageData struct {
 	PageNumbers         []PageNumber
 	ShowHeader          bool
 	IsImageSearch       bool
+	// Theme is the user's selected theme (light/dark) from cookie
+	Theme              string
 	// New fields for three-column layout
 	Categories         []string
 	CategoryIcons     map[string]string
@@ -280,9 +282,9 @@ func FromResponse(resp contracts.SearchResponse, query string, pageno int, activ
 }
 
 // RenderIndex renders the homepage (search box only).
-func RenderIndex(w http.ResponseWriter, sourceURL string) error {
+func RenderIndex(w http.ResponseWriter, sourceURL, theme string) error {
 	w.Header().Set("Content-Type", "text/html; charset=utf-8")
-	return tmplIndex.ExecuteTemplate(w, "base", PageData{ShowHeader: true, SourceURL: sourceURL})
+	return tmplIndex.ExecuteTemplate(w, "base", PageData{ShowHeader: true, SourceURL: sourceURL, Theme: theme})
 }
 
 // RenderSearch renders the full search results page (with base layout).
@@ -326,8 +328,8 @@ func RenderSearchAuto(w http.ResponseWriter, r *http.Request, data PageData) err
 }
 
 // RenderPreferences renders the full preferences page.
-func RenderPreferences(w http.ResponseWriter, sourceURL string) error {
+func RenderPreferences(w http.ResponseWriter, sourceURL, theme string) error {
 	w.Header().Set("Content-Type", "text/html; charset=utf-8")
-	return tmplPreferences.ExecuteTemplate(w, "base", PageData{ShowHeader: true, SourceURL: sourceURL})
+	return tmplPreferences.ExecuteTemplate(w, "base", PageData{ShowHeader: true, SourceURL: sourceURL, Theme: theme})
 }