feat: add UI prototype for Connect Associated Datasets feature

[sven1103]

Adds a development-only demo view at /dev/demo/associated-datasets that covers all 8 user stories from the 'Connect Associated Datasets' feature spec (2026-06-19):

• Story 01/05: Search & connect open/restricted datasets via a large dialog with InvenioRDM instance selector, live-filtered search field, and single-select result grid
• Story 02/06: View connected datasets in two Section components (Public Datasets / Restricted Datasets) each backed by a grid showing title, clickable DOI/access link, repository tag, version, connected-by user + date
• Story 03/07: Remove connection via a danger-confirm dialog
• Story 04/08: Sync single dataset (row icon) and Sync All (action bar button) with simulated success/info notifications
• Story 05 extra: dismissible InfoBox warning when opening the restricted search if no access token is configured

All interactions are simulated in-memory; no backend is contacted. The view is gated behind @Profile("development") and @AnonymousAllowed.

Also fixes two missing CSS rules surfaced during visual review:

• .actionbar: flex-direction row, nowrap, gap — buttons were stacking vertically because the class had no rule in the theme
• .section-header-row: adds justify-content space-between and align-items center so title and action bar sit on opposite ends
• .tag.teal: adds background/colour for the FDAT repository tag (TagColor.TEAL was referencing a non-existent CSS class)

Description

Issue and Traceability

Linked Task Issue

• Closes: #

Requirement IDs Addressed

• Requirement IDs:

Requirement Update Status

• Requirements Updated: The corresponding functional/non-functional requirements in docs/requirements.md have been updated to reflect this implementation.
• Justification Provided: This PR does not change externally observable behavior, so no requirement update is needed. (Provide brief justification below)

Justification (if applicable)

Changes Summary

Modified Areas

• Domain layer (domain/model/, domain events, aggregates)
• Application layer (application/ services, use cases)
• Infrastructure layer (JPA repositories, external integrations, config)
• UI / Views (views/ components, Vaadin routes)
• Database schema (sql/, DDL changes)
• Tests (unit, integration, or Spock specs)
• Documentation, CI/CD, or build configuration
• Other (describe):

Behavioral Changes

• No behavioral changes (internal refactoring, optimization, test addition, etc.)
• Yes, behavior changes:

Sensitive Changes Requiring Review

• Database schema changes (sql/complete-schema.sql, table/column DDL)
• Spring Security configuration (security/, ACL setup, authentication/authorization)
• FAIR / RO-Crate export format (docs/fair/, RO-Crate builder logic)
• Artemis messaging topics (JMS topic names, consumer configuration)
• Requirement file edits (docs/requirements.md)
• None of the above

Pre-Submission Checklist

• Issue Linked: PR references a Task issue (see "Linked Task Issue" section above)
• Requirements Listed: All functional (R) and non-functional (NFR) requirement IDs are listed above
• No Constraint IDs: No C (constraint) IDs are listed in the Requirement IDs section
• Behavior vs. Requirements: Requirement update confirmed OR explicit justification provided
• Code Style: Code follows Google Java Style Guide (run formatter before commit)
• Tests: New tests added or existing tests updated to cover changes
• No Secrets: No hardcoded credentials, API keys, passwords, or secrets committed
• Documentation: Updated relevant docs if the change affects user-facing behavior or public APIs

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Notes for Reviewers