feat(render): full-screen render page, one-active-render limit, app-wide progress

Concurrent-render ceiling (a user runs 1 render at a time unless granted more):
- Identity: TokenService emits max_renders claim from User.ParallelRenderingCeiling
- Identity: admin POST /v1/users/{id}/render-slots (AdminService.SetRenderSlotsAsync,
  clamped 1..50) — gamification or admin raises a user's ceiling
- render-svc: middleware reads max_renders (default 1); CreateJob rejects with 409
  active_render_limit when active jobs >= ceiling
- render-svc: db.CountActiveJobs + ListActiveJobs; GET /v1/renders/active returns
  in-flight renders + can_start_new

Full-screen render page (replaces the modal):
- /studio/render/[projectId]: config (resolution/fps) → live preview + progress →
  download; resumes this project's in-flight render on mount; blocks when another
  render is active; reads ?preset=
- StudioTopBar export menu now navigates to the page; RenderModal deleted (dead)

App-wide minimal progress:
- GlobalRenderProgress pill mounted in the locale layout for authed users; polls
  /api/render/active every 4s, shows thumbnail + step + % on every page, click →
  the render page; hidden on the render page and when idle

Admin: UserActions gains a "concurrent render slots" control.

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

services/render/cmd/server/main.go

@@ -94,6 +94,7 @@ func main() {
 	{
 		renders.GET("", renderH.List)
 		renders.POST("", renderH.Create)
+		renders.GET("/active", renderH.Active)
 		renders.GET("/:job_id", renderH.Get)
 		renders.POST("/:job_id/cancel", renderH.Cancel)
 		renders.POST("/:job_id/stop", admin, renderH.Stop)