Fix docked sidebar pane tab alignment

[hamzamerzic]

Summary

• scope the docked-sidebar offset to direct shell layout children
• keep nested multi-pane tab strips in the same projected coordinate system as pane content
• add selector-contract and browser geometry regressions for sidebar open, collapsed, and reopened states

Validation

• frontend unit and hook suite: 1,430 passed
• production and PWA build passed
• shell/workspace focused suite: 36 passed after final rebase
• isolated tabs and desktop-sidebar browser matrix: 9 passed on the exact rebased commit
• diff and pre-push gates passed