refactor(26088): Refactor frontend to align with new mapping paradigm

[vanch3d]

See https://hivemq.kanbanize.com/ctrl_board/57/cards/26088/details/

This PR is part of the bi-directional epic and aligns the codebase with the significant refactoring of the "subscription" into a "mapping" paradigm, see https://hivemq.kanbanize.com/ctrl_board/57/cards/24918/details/

Design

• The discovery of topics has been refactored, based on the new (undocumented) structure for mappings
• The use of "subscription" for user-facing strings in the context of adapters has been changed
• The use of "subscription" in the codebase related to adapters has been changed

Out-of-scope

• The "Export Mappings" functionality is designed for a single (inward) source. Extending it to bi-directionality (and fixing the design flaws) will be done in a subsequent ticket
• The discovery of topics is still based on reverse-traversing the schema from the mqtt-topic formats, creating uncertainties when supporting bi-directional mappings. A better traversing algorithm will be implemented in a further ticket