fix: protect share accept endpoint with CSRF token

[kristian-zendato]

• Resolves: https://github.com/nextcloud-gmbh/h1/issues/252

Summary

The share accept link in notification emails previously accepted the share on a single GET request, which bypass CSRF protection. This change introduces a confirmation page and moves the actual acceptance to a POST request with a CSRF token.

• GET /apps/files_sharing/accept/{shareId} — shows a confirmation page with sharer name and file name
• POST /apps/files_sharing/accept/{shareId} — accepts the share and redirects to the shared file (CSRF-protected)
• Share notification emails now link to the confirmation page (showAccept) instead of accepting directly

Screenshot of Accept page

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[susnux]

--- Failed scenarios:

/home/runner/work/server/server/build/integration/sharing_features/sharing-accept.feature:20 (on line 32)

related error

[susnux]

/backport to stable35

[joshtrichards]

Might be a good idea to add a negative regression test for GET behavior as a followup.

[susnux]

/backport to stable34

[susnux]

/backport to stable33

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