fix(ai-create): sync the prompt limit across front and back

The counter added earlier drifted from the backend in two ways: it counted
UTF-16 code units over the raw (untrimmed) value, while the backend measures
Unicode characters (mb_strlen) over the trimmed value that is actually sent —
so emoji or trailing whitespace could falsely turn the counter red and block
the button. The 2000 limit was also copied into three places, and the wizard's
frontend `>= 3` minimum had no backend counterpart.

- Add App\Support\AiPromptRules as the single source of truth for the prompt
  bounds; both StartPostCreationRequest and GeneratePostContentRequest use it.
- Add min:3 to the create wizard endpoint so front and back agree (the editor's
  generate-content flow keeps `required` — it has no counter to mirror).
- Count code points over the trimmed value in AiPostWizard so the counter and
  the submit gate match what the backend validates, matching AltTextDialog.
- Cover min/max/boundary in PostAiCreateTest.

app/Http/Requests/App/Ai/GeneratePostContentRequest.php

@@ -4,6 +4,7 @@
 
 namespace App\Http\Requests\App\Ai;
 
+use App\Support\AiPromptRules;
 use Illuminate\Foundation\Http\FormRequest;
 
 class GeneratePostContentRequest extends FormRequest
@@ -19,7 +20,7 @@ public function authorize(): bool
     public function rules(): array
     {
         return [
-            'prompt' => ['required', 'string', 'max:2000'],
+            'prompt' => ['required', 'string', 'max:'.AiPromptRules::PROMPT_MAX_LENGTH],
             'current_content' => ['nullable', 'string', 'max:10000'],
         ];
     }

app/Http/Requests/App/Ai/StartPostCreationRequest.php

@@ -6,6 +6,7 @@
 
 use App\Enums\Ai\ContentStyle;
 use App\Enums\PostPlatform\ContentType;
+use App\Support\AiPromptRules;
 use Illuminate\Foundation\Http\FormRequest;
 use Illuminate\Validation\Rule;
 use Illuminate\Validation\Validator;
@@ -33,7 +34,7 @@ public function rules(): array
             ],
             'social_account_id' => ['nullable', 'uuid'],
             'image_count' => ['nullable', 'integer', 'min:0', 'max:10'],
-            'prompt' => ['required', 'string', 'max:2000'],
+            'prompt' => AiPromptRules::promptRule(),
             'date' => ['nullable', 'date_format:Y-m-d'],
             'template' => ['sometimes', 'string', Rule::enum(ContentStyle::class)],
         ];

app/Support/AiPromptRules.php

@@ -0,0 +1,34 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Support;
+
+/**
+ * Single source of truth for the user-supplied AI generation prompt length,
+ * shared by the create wizard (StartPostCreationRequest) and the editor's
+ * content generation (GeneratePostContentRequest). The frontend counter in
+ * resources/js/components/posts/create/AiPostWizard.vue mirrors these bounds.
+ */
+class AiPromptRules
+{
+    /**
+     * Minimum prompt length (characters) for the create wizard.
+     */
+    public const PROMPT_MIN_LENGTH = 3;
+
+    /**
+     * Maximum prompt length (characters); mirrored by the frontend counter.
+     */
+    public const PROMPT_MAX_LENGTH = 2000;
+
+    /**
+     * Validation rules for the create wizard's generation prompt.
+     *
+     * @return array<int, string>
+     */
+    public static function promptRule(): array
+    {
+        return ['required', 'string', 'min:'.self::PROMPT_MIN_LENGTH, 'max:'.self::PROMPT_MAX_LENGTH];
+    }
+}

resources/js/components/posts/create/AiPostWizard.vue

@@ -63,7 +63,7 @@ const selectedAccountId = ref<string | null>(null);
 const includeImages = ref(true);
 const imageCount = ref(2);
 const promptText = ref('');
-// Mirrors the backend validation (`prompt` max:2000) so the limit is visible in the UI.
+const PROMPT_MIN = 3;
 const PROMPT_MAX = 2000;
 
 const submitting = ref(false);
@@ -159,11 +159,13 @@ const submittedImageCount = computed(() => {
     return 0;
 });
 
+const promptLength = computed(() => [...promptText.value.trim()].length);
+
 const canSubmit = computed(() =>
     selectedFormat.value !== null &&
     selectedAccountId.value !== null &&
-    promptText.value.trim().length >= 3 &&
-    promptText.value.length <= PROMPT_MAX,
+    promptLength.value >= PROMPT_MIN &&
+    promptLength.value <= PROMPT_MAX,
 );
 
 // Auto-pick the only account when format has exactly one match.
@@ -363,9 +365,9 @@ const startGeneration = async () => {
             />
             <p
                 class="text-right text-xs tabular-nums"
-                :class="promptText.length > PROMPT_MAX ? 'font-semibold text-destructive' : 'text-muted-foreground'"
+                :class="promptLength > PROMPT_MAX ? 'font-semibold text-destructive' : 'text-muted-foreground'"
             >
-                {{ promptText.length }}/{{ PROMPT_MAX }}
+                {{ promptLength }}/{{ PROMPT_MAX }}
             </p>
         </div>
 

tests/Feature/Ai/PostAiCreateTest.php

@@ -8,6 +8,7 @@
 use App\Models\SocialAccount;
 use App\Models\User;
 use App\Models\Workspace;
+use App\Support\AiPromptRules;
 use Illuminate\Support\Facades\Bus;
 use Symfony\Component\HttpFoundation\Response;
 
@@ -32,6 +33,44 @@
         ->assertJsonValidationErrors(['prompt']);
 });
 
+test('start rejects a prompt shorter than the minimum length', function () {
+    Bus::fake();
+
+    $this->actingAs($this->user)
+        ->postJson(route('app.posts.ai.create'), ['prompt' => 'hi', 'format' => 'x_post'])
+        ->assertStatus(Response::HTTP_UNPROCESSABLE_ENTITY)
+        ->assertJsonValidationErrors(['prompt']);
+
+    Bus::assertNotDispatched(StreamPostCreation::class);
+});
+
+test('start rejects a prompt longer than the maximum length', function () {
+    Bus::fake();
+
+    $this->actingAs($this->user)
+        ->postJson(route('app.posts.ai.create'), [
+            'prompt' => str_repeat('a', AiPromptRules::PROMPT_MAX_LENGTH + 1),
+            'format' => 'x_post',
+        ])
+        ->assertStatus(Response::HTTP_UNPROCESSABLE_ENTITY)
+        ->assertJsonValidationErrors(['prompt']);
+
+    Bus::assertNotDispatched(StreamPostCreation::class);
+});
+
+test('start accepts a prompt at the maximum length', function () {
+    Bus::fake();
+
+    $this->actingAs($this->user)
+        ->postJson(route('app.posts.ai.create'), [
+            'prompt' => str_repeat('a', AiPromptRules::PROMPT_MAX_LENGTH),
+            'format' => 'x_post',
+        ])
+        ->assertStatus(Response::HTTP_ACCEPTED);
+
+    Bus::assertDispatched(StreamPostCreation::class);
+});
+
 test('start validates format is required', function () {
     Bus::fake();