Unify thread and sidebar-tab full screen

[brsbl]

The top stack layer makes thread and sidebar-tab full screen use one clean expand/collapse pattern while preserving the prior layout. It also restores a visible shared selection surface for sidebar threads, focused split tabs, and active right-panel tabs.

Changes

• Keep the sidebar-tab expand/collapse control in the panel header in both states, with Full Screen and Exit Full Screen copy.
• Remove the collapsed-thread rail and let a full-screen sidebar tab occupy the complete content area.
• Suppress the hosted right panel while a split thread is full screen, then restore its prior open/full-screen layout on exit.
• Align the panel expand/collapse icon geometry with the pane-header control.
• Consolidate sidebar-thread, focused split-tab, and active right-panel-tab selection onto the shared bg-state-active surface.

Verification

• Focused @bb/app suites: 71 tests passed across 9 files.
• Turbo @bb/app typecheck passed.
• BB desktop QA passed for sidebar-tab expand/collapse, thread expand/collapse, prior-layout restoration, clean full-screen surfaces, icon placement, and Full Screen / Exit Full Screen labels.
• Light and dark QA confirmed the three shared selection states resolve to identical computed colors in each theme.

Stack

Native GitHub stack #960: #941 → #953 → #954 → #959.