fix(posts): drop redundant 'publishing' toast on publish

[paulocastellano]

Why

When you publish a post, you land on Show.vue which already renders a full-screen overlay (`isPublishing` block — spinner + 'Seu post está sendo publicado' + subtitle) while `post.status === 'publishing'`. The flash toast was saying the same thing transiently, adding visual noise.

Compounding this: each platform job emits 2+ Echo broadcasts during publish (`PostPlatformStatusUpdated`), and Show.vue's listener does `router.visit({ only: ['post'] })` per event. Multiple partial reloads make the page feel busy. Removing the toast won't fix the over-broadcasting (that's a separate concern — see below), but it removes the redundant signal that was the visible irritation.

Changes

• `PostController::update` drops `session()->flash('flash.banner', ...)` for the `Publishing` action. The redirect to `app.posts.show` is unchanged.
• Drop the orphan `posts.flash.publishing` key from `lang/en|pt-BR|es/posts.php`.
• `Scheduled` action keeps its flash — no overlay equivalent on Show.vue.

Follow-up (not in this PR)

The over-broadcasting in `PublishToSocialPlatform` (6 `broadcastStatus()` call sites, ~2 broadcasts per platform in the happy path) causes N partial reloads on the frontend per publish. Fix ideas:

1. Include status + url + error in `PostPlatformStatusUpdated::broadcastWith()` so the frontend patches local state instead of reloading.
2. Reduce the broadcast cadence to just the terminal state per platform.
3. Debounce `router.visit` on the Echo listener.

Notifications module is already gated to fire once per post (only when all platforms finish — see `PublishToSocialPlatform::updatePostStatus` lines 217-219), so that part is fine.

Test plan

• Full suite: 1542 passing
• Publish a post → land on Show.vue → overlay shows, no toast
• Schedule a post → land on Show.vue → toast shows (unchanged behavior)
• Grep clean: no other `posts.flash.publishing` references