UX: Add "Success Celebration" Modal

[marchange]

Description: Instead of just changing the button text, implement a dedicated "Success Modal" (pop-up) that appears after the tree is planted. It should feature a "Share on LinkedIn" button to help your project go viral.

[randomgituser9]

Implemented — SubmitForm.vue shows a proper modal (Teleport'd to body, backdrop blur, Esc/click-outside to close) after a successful submit, with a 'Auf LinkedIn teilen' button linking to the new /certificate/:id permalink page.