feat: add new page and route for sponsor customized form managed items

[tomrndom]

ref:

• https://app.clickup.com/t/86b7e0rkb
• https://app.clickup.com/t/86b66n5jp
• https://app.clickup.com/t/86b66n5jy
• https://app.clickup.com/t/86b7km2c3
• https://app.clickup.com/t/86b66n5ju
• https://app.clickup.com/t/86b66n5jr

Signed-off-by: Tomás Castillo tcastilloboireau@gmail.com

Summary by CodeRabbit

• New Features

  • Full sponsor form item management UI: browse/import from inventory, create/edit/clone/delete, archive/unarchive, bulk add, image preview, inline price editing, pagination, sorting, search, and dialogs.

• Localization

  • Added English UI strings for form item management (labels, buttons, alerts, success messages, placeholders).

• Routing

  • Added Manage Items sub-route and routing-aware default tab behavior.

• Validation

  • Centralized currency/rate validation and parsing for consistent input handling.

• State

  • New persisted state slice for sponsor customized form items.

• UX Fixes

  • Improved dialog titles, image label, and minor form behaviours.

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