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>

src/hooks/useStudioProjectPersistence.ts

@@ -12,8 +12,10 @@ import {
   contentValuesFromSceneData,
   fetchProject,
   isProjectNotFoundError,
+  patchProjectColors,
   patchProjectContents,
   patchProjectSceneData,
+  themeColorsFromSceneData,
 } from "@/lib/project-api";
 import { isDevProjectId } from "@/lib/project-ids";
 import {
@@ -159,6 +161,12 @@ export function useStudioProjectPersistence(
           projectId,
           contentValuesFromSceneData(scene_data)
         ).catch(() => {});
+        // Persist the theme colours to saved_shared_colors so the FlexStory render
+        // reads the theme picker (best-effort).
+        void patchProjectColors(
+          projectId,
+          themeColorsFromSceneData(scene_data)
+        ).catch(() => {});
         setUsingLocalStorage(false);
         markSaved(payloadJson, false);
       } catch (error) {