docs: move CSS cache warming to completed (already implemented)

The CSSCache.init/1 already calls warm() on startup.
Remove from Quick Wins, add to Completed section.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ROADMAP.md

@@ -339,23 +339,6 @@ This ensures sellers never unknowingly sell at a loss due to Printify price chan
 
 ## Quick Wins (Low Effort)
 
-### CSS Cache Warming on Startup
-**Status:** Not implemented
-**Effort:** Small
-
-Currently the CSS cache (ETS) is created on startup but not pre-warmed. The first request after server restart generates CSS on-demand.
-
-**Implementation:**
-Add cache warming to `lib/simpleshop_theme/application.ex`:
-```elixir
-# After supervisor starts
-Task.start(fn ->
-  settings = SimpleshopTheme.Settings.get_theme_settings()
-  css = SimpleshopTheme.Theme.CSSGenerator.generate(settings)
-  SimpleshopTheme.Theme.CSSCache.put(css)
-end)
-```
-
 ### Navigation Links Between Admin and Shop
 **Status:** Not implemented
 **Effort:** Small
@@ -501,3 +484,7 @@ The project is currently named `simpleshop_theme` (reflecting its origins as a t
 - CSS injection via shop layout
 - Themed error pages (404/500)
 - Dev routes for error page preview
+
+### CSS Cache Warming on Startup ✅
+- ETS cache pre-warmed in `CSSCache.init/1`
+- First request doesn't need to generate CSS