refactor(homepage): reorganize icons into dedicated package structure

Split the monolithic `internal/homepage` icons functionality into a structured package hierarchy:

- `internal/homepage/icons/` - Core types (URL, Key, Meta, Provider, Source, Variant)
- `internal/homepage/icons/fetch/` - Icon fetching logic (content.go, fetch.go, route.go)
- `internal/homepage/icons/list/` - Icon listing and search (list_icons.go, list_icons_test.go)

Moved icon-related code from `internal/homepage/`:
- `icon_url.go` → `icons/url.go` (+ url_test.go)
- `content.go` → `icons/fetch/content.go`
- `route.go` → `icons/fetch/route.go`
- `list_icons.go` → `icons/list/list_icons.go` (+ list_icons_test.go)

Updated all consumers to use the new package structure:
- `cmd/main.go`
- `internal/api/v1/favicon.go`
- `internal/api/v1/icons.go`
- `internal/idlewatcher/handle_http.go`
- `internal/route/route.go`

internal/homepage/homepage_test.go

@@ -4,19 +4,24 @@ import (
 	"testing"
 
 	. "github.com/yusing/godoxy/internal/homepage"
+	"github.com/yusing/godoxy/internal/homepage/icons"
 
 	expect "github.com/yusing/goutils/testing"
 )
 
+func strPtr(s string) *string {
+	return &s
+}
+
 func TestOverrideItem(t *testing.T) {
 	a := &Item{
 		Alias: "foo",
 		ItemConfig: ItemConfig{
 			Show: false,
 			Name: "Foo",
-			Icon: &IconURL{
-				FullURL:    strPtr("/favicon.ico"),
-				IconSource: IconSourceRelative,
+			Icon: &icons.URL{
+				FullURL: strPtr("/favicon.ico"),
+				Source:  icons.SourceRelative,
 			},
 			Category: "App",
 		},
@@ -25,9 +30,9 @@ func TestOverrideItem(t *testing.T) {
 		Show:     true,
 		Name:     "Bar",
 		Category: "Test",
-		Icon: &IconURL{
-			FullURL:    strPtr("@walkxcode/example.png"),
-			IconSource: IconSourceWalkXCode,
+		Icon: &icons.URL{
+			FullURL: strPtr("@walkxcode/example.png"),
+			Source:  icons.SourceWalkXCode,
 		},
 	}
 	overrides := GetOverrideConfig()