fix(dashboard): form reset for across allocation (#14369)

**What**
- fix an issue where switching between `across` promotion types in the create form wouldn't set `max_quantity` to null which would cause the create request to fail

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CLOSES https://github.com/medusajs/medusa/issues/14365


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> [!NOTE]
> Prevents invalid `max_quantity` when switching templates to an *across* allocation in the promotion creation flow.
> 
> - In `create-promotion-form.tsx`, after applying template defaults, explicitly sets `application_method.max_quantity` to `null` if `application_method.allocation` is `"across"` to avoid resetting it to `1`
> - Adds a changeset entry for `@medusajs/dashboard` patch release
> 
> <sup>Written by [Cursor Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit 5b08c77fee741ec5de399b4f1f9e245ba69b5bee. This will update automatically on new commits. Configure [here](https://cursor.com/dashboard?tab=bugbot).</sup>


Co-authored-by: Oli Juhl <59018053+olivermrbl@users.noreply.github.com>

.changeset/eight-baths-win.md

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+---
+"@medusajs/dashboard": patch
+---
+
+fix(dashboard): form reset for across allocation