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/icons/provider.go

@@ -0,0 +1,21 @@
+package icons
+
+import "sync/atomic"
+
+type Provider interface {
+	HasIcon(u *URL) bool
+}
+
+var provider atomic.Value
+
+func SetProvider(p Provider) {
+	provider.Store(p)
+}
+
+func hasIcon(u *URL) bool {
+	v := provider.Load()
+	if v == nil {
+		return false
+	}
+	return v.(Provider).HasIcon(u)
+}