Update URL structure

[XanderVertegaal]

Closes #228

Moves the start page to /start and the form pages to /studies/studyId/submissionId/slug.

[Copilot]

Pull request overview

Updates the frontend routing to support the new form URL structure requested in #228, moving form navigation under /studies/[studyId]/[submissionId]/[slug] and introducing shared helpers for route params.

Changes:

• Add /studies/[study_id]/[submission_id]/[slug] page and update form navigation URLs to include studyId.
• Introduce useRouteParams composable for consistent route param coercion.
• Add new /start page and update the navbar “New registration” link accordingly.

Reviewed changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
frontend/pages/studies/[study_id]/index.vue	Uses useStudyId() and updates the study query wiring for the new route param handling.
frontend/pages/studies/[study_id]/[submission_id]/[slug].vue	Switches to useSubmissionId()/useStepSlug() and gates rendering on slug presence.
frontend/pages/start.vue	Adds new “start new registration” page that creates a study and routes to /studies/:id.
frontend/composables/useRouteParams.ts	Adds reusable helpers for route param normalization.
frontend/components/study_detail/AvailableActions.vue	Updates “Continue editing” URL to the new studies-based route and switches tiles to NuxtLink.
frontend/components/shared/FormWrapper.vue	Updates step navigation to the new studies-based URL format.
frontend/components/layout/NavBar.vue	Updates “New registration” nav link to /start.

Comments suppressed due to low confidence (1)

frontend/pages/studies/[study_id]/index.vue:7

• GetStudyQueryVariables is imported but never used. This will typically trip lint/typecheck in this repo; either remove the import or use it as the second generic for useQuery (e.g., useQuery<GetStudyQuery, GetStudyQueryVariables>(...)) so the variables function is type-checked.

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[EdoStorm96]

While I think this is a good idea, I would prefer folder names to be camelCase i/o snake_case (even though I started doing it in snake_case ... It turns out this is not ideal ... Could you change it to this?

With AvailableActions, there will be some merge issues with #233, but we'll cross that bridge when we get there ...