Keep frontend delivery and streaming rendering responsive

[VolkerChristian]

Summary

• retain the two-row, equal-width turn-settings layout and the inherent full-access network policy presentation
• move frontend transport and AISuite processing behind a dedicated worker with a bounded latest-state mailbox
• preserve lifecycle and completion ordering while coalescing superseded state publications
• render streaming content incrementally as plain text and promote it once to final sanitized Markdown
• update layouts only for real geometry changes, pin the tail directly, and freeze off-bottom presentation while new content arrives
• keep incomplete-thread recovery and reconnect handling bounded and stable

Validation

• complete fresh Release CTest suite: 7/7 passed
• isolated live gate passed across a backend stop/restart while the exact same CodexUI process remained alive, reconnected, and resynchronized
• no backpressure, broken-pipe, capacity, escaped-callback, or protocol-encoding diagnostics in the gate
• final worker, mailbox, rendering, scroll, lifetime, and reconnect audit found no blockers