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/Application/Services/StudioService.cs

@@ -321,6 +321,40 @@ public class StudioService(StudioDbContext db)
         return updated;
     }
 
+    /// <summary>
+    /// Update the project's theme colours by element key (accentColor/secondaryColor/
+    /// backgroundColor/textColor). Upserts saved_shared_colors so the FlexStory render
+    /// binder (GetFlexStoryProps) reads the studio theme picker's colours.
+    /// </summary>
+    public async Task<int> UpdateSharedColorsAsync(Guid projectId, Guid userId, List<UpdateColorItem> items)
+    {
+        var owns = await db.SavedProjects.AnyAsync(x => x.Id == projectId && x.UserId == userId);
+        if (!owns) throw new KeyNotFoundException($"Project {projectId} not found");
+
+        var updated = 0;
+        foreach (var item in items)
+        {
+            if (string.IsNullOrWhiteSpace(item.Key) || string.IsNullOrWhiteSpace(item.Value)) continue;
+            // Upsert by (project, element_key): the row usually exists (copied from the
+            // template's shared colours); insert if a FlexStory key is missing.
+            var n = await db.Database.ExecuteSqlInterpolatedAsync($@"
+                UPDATE studio.saved_shared_colors
+                SET value = {item.Value}
+                WHERE saved_project_id = {projectId} AND element_key = {item.Key}");
+            if (n == 0)
+            {
+                n = await db.Database.ExecuteSqlInterpolatedAsync($@"
+                    INSERT INTO studio.saved_shared_colors
+                        (saved_project_id, element_key, attr_value, value, is_selected, sort)
+                    VALUES ({projectId}, {item.Key}, 'fill', {item.Value}, true, 0)");
+            }
+            updated += n;
+        }
+        await db.Database.ExecuteSqlInterpolatedAsync(
+            $"UPDATE studio.saved_projects SET last_edit_date = now(), updated_at = now() WHERE id = {projectId}");
+        return updated;
+    }
+
     public async Task DeleteProjectAsync(Guid id, Guid userId)
     {
         var project = await db.SavedProjects

services/studio/FlatRender.StudioSvc/Controllers/StudioController.cs

@@ -60,6 +60,12 @@ public class StudioController(StudioService svc) : ControllerBase
     public async Task<IActionResult> UpdateContents(Guid id, [FromBody] UpdateContentsRequest req) =>
         Ok(new { updated = await svc.UpdateSceneContentsAsync(id, UserId, req.Items ?? new List<UpdateContentItem>()) });
 
+    /// <summary>Update the project's theme colours (accentColor/secondaryColor/
+    /// backgroundColor/textColor) so the FlexStory render reads the theme picker.</summary>
+    [HttpPatch("{id:guid}/shared-colors")]
+    public async Task<IActionResult> UpdateColors(Guid id, [FromBody] UpdateColorsRequest req) =>
+        Ok(new { updated = await svc.UpdateSharedColorsAsync(id, UserId, req.Items ?? new List<UpdateColorItem>()) });
+
     /// <summary>Internal endpoint: get project for render service (no user-ownership check).</summary>
     [HttpGet("{id:guid}/render-payload")]
     [Authorize(Roles = "Service")]

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);