closes #3075 
Implements pause/resume functionality for the message queue processing
system, allowing users to pause automatic processing of queued messages
and selectively resume. Includes session-level persistence via
sessionStorage.


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9 comprehensive E2E test cases covering:
Pause/Resume button visibility
Pause functionality and message blocking
Resume processing
SessionStorage persistence (empty & non-empty cases)
Multiple pause/resume cycles
Per-chat isolation
Visual styling