feat(ui): dark theme redesign, fix image search and defaults

- Inline CSS in base.html (Inter, dark mode, sticky search, tabs, results)
- Remove HTMX/JS from templates; pagination via GET links
- Atmospheric side gradients + grid; wider column on large viewports
- Parse ?category= for HTML tabs (fixes Images category routing)
- Include bing_images, ddg_images, qwant_images in local_ported defaults
- Default listen port 5355; update Docker, compose, flake, README
- Favicon img uses /favicon/ proxy; preferences without inline JS

Made-with: Cursor

internal/search/request_params_test.go

@@ -72,3 +72,21 @@ func TestParseSearchRequest_CategoriesAndEngineData(t *testing.T) {
 	}
 }
 
+func TestParseSearchRequest_SingularCategoryParam(t *testing.T) {
+	r := httptest.NewRequest(http.MethodGet, "/search?q=cats&category=images", nil)
+	req, err := ParseSearchRequest(r)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	found := false
+	for _, c := range req.Categories {
+		if c == "images" {
+			found = true
+			break
+		}
+	}
+	if !found {
+		t.Fatalf("expected category images from ?category=images, got %v", req.Categories)
+	}
+}
+