Add Cross-Posting support for multiple social accounts

- Create post_social_account pivot table for many-to-many relationship
- Add migration to migrate existing social_account_id to pivot table
- Remove social_account_id column from posts table
- Update Post model with socialAccounts() BelongsToMany relationship
- Update Posts Livewire component to use array of selectedAccountIds
- Update Posts view with checkboxes for selecting multiple accounts
- Update PublishPostJob to iterate through all social accounts of matching type
- Update Publisher interface to accept optional SocialAccount parameter
- Update all Publishers (WordPress, Reddit, Twitter, LinkedIn) to support cross-posting
- Display multiple social accounts as badges in posts table

Now a single post can be published to multiple social accounts of the same type.
For example: a Twitter post can be published to multiple Twitter accounts.

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

app/Support/Publishing/TwitterPublisher.php

@@ -13,13 +13,14 @@ class TwitterPublisher implements Publisher
         return $post->type === 'twitter';
     }
 
-    public function publish(Post $post): PublishResult
+    public function publish(Post $post, ?SocialAccount $socialAccount = null): PublishResult
     {
         if (! $post->body) {
             return PublishResult::rejected("Post {$post->id} has no body, required for a Twitter post.");
         }
 
-        $account = $post->socialAccount;
+        // Use the passed social account, or fall back to the first linked one
+        $account = $socialAccount ?: $post->socialAccounts->first();
 
         if (! $account || $account->network !== 'twitter') {
             return PublishResult::rejected("Post {$post->id} has no linked Twitter social account.");