feat(ai-create): global 'AI is generating…' bar that persists across navigation

[dantaspaulo]

Problem

AI generation runs on the backend — the StreamPostCreation job broadcasts
.ai.creation.completed on the private user.{id}.ai-creation.{creationId}
channel — but only the dedicated loading screen listened to that channel. As
soon as the user navigated away, they lost any visual sign that a generation was
still running; only the final bell notification told them.

Change

A global singleton composable, useAiGeneration, becomes the sole owner of
the Echo subscription. Its module-level state persists across Inertia
navigations (SPA) and survives a hard reload via sessionStorage — it
re-subscribes to still-running generations, with a safety timeout so the bar
never gets stuck.

A thin bar at the top of the content shows "AI is generating your post…". On
completion it turns into a clickable "Your post is ready — View post"; on failure
it becomes a dismissible error.

The loading screen no longer subscribes to Echo directly — it now observes
the composable and redirects when its generation is done. That also removes the
two-owners conflict of both the screen and the composable calling leave() on
the same channel.

How

• useAiGeneration.ts (new) — owns the channel subscription and the shared
  state (track, hydrate, complete, per-id find).
• AiGenerationBar.vue (new) — the bar, mounted at the top of SidebarInset in
  AppSidebarLayout.
• Loading.vue — observes the composable instead of Echo; owns no subscription.
• lang/{en,es,pt-BR}/posts.php — bar_* strings.

The channel name, event (.ai.creation.completed) and payload
({ post_id, error }) are unchanged — the same contract the loading screen
already used.