feat(studio+render): wire theme picker → saved_shared_colors → FlexStory render

Closes the theme→render gap: the studio theme picker now actually drives a
FlexStory render's colours. GetFlexStoryProps reads saved_shared_colors by
element_key (accentColor/secondaryColor/backgroundColor/textColor), but the studio
only wrote the theme into scene_data — so the picker never reached the MP4.

- studio-svc: UpdateSharedColorsAsync upserts saved_shared_colors by (project,
  element_key) + PATCH /v1/saved-projects/{id}/shared-colors endpoint +
  UpdateColorsRequest/UpdateColorItem. Mirrors UpdateSceneContentsAsync. (dotnet
  build: 0 errors.)
- gateway already wildcard-routes /v1/saved-projects/*path → studio-svc (no change).
- Next: /api/projects/[id]/colors route → gateway; project-api patchProjectColors
  + themeColorsFromSceneData (maps scene_data sceneAccentColor… → the colorSchema
  keys); performSave best-effort pushes the 4 colours alongside contents.

Chain: theme picker → store → scene_data → performSave → patchProjectColors →
gateway → studio-svc upsert → saved_shared_colors → GetFlexStoryProps → render.
Verified: Next build + dotnet build both clean; theme presets render cohesively
across all 6 (incl. dark Midnight). End-to-end studio→render needs the live stack.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/studio/FlatRender.StudioSvc/Models/Requests/Requests.cs

@@ -128,3 +128,9 @@ public record SavedProjectListRequest(
 /// writes the user's edits here (by content key) so the render binder picks them up.</summary>
 public record UpdateContentsRequest(List<UpdateContentItem> Items);
 public record UpdateContentItem(string Key, string? Value, Guid? ValueFileId);
+
+/// <summary>Update the project-wide theme colours (the studio theme picker) by
+/// element key (accentColor/secondaryColor/backgroundColor/textColor) so the
+/// FlexStory render binder reads them from saved_shared_colors.</summary>
+public record UpdateColorsRequest(List<UpdateColorItem> Items);
+public record UpdateColorItem(string Key, string Value);