Fix: stabilize mobile sidebar and header spacing

[xIGBClutchIx]

Summary

This fixes the recent mobile sidebar regressions introduced by newer Codex layout behavior and also corrects the header icon spacing shift in Pocodex.

What changed

• add a mobile sidebar toggle fallback when the native toggle click no longer dispatches the expected sidebar state change
• detect mobile sidebar open/closed state from the actual sidebar shell width instead of the inner navigation geometry
• close the mobile sidebar on text-entry touchstart so the composer focus + keyboard transition does not race the sidebar overlay
• reset the header action group start padding to the browser safe-area inset so the top icons stay aligned again

Root cause

Recent Codex builds changed both the sidebar layout behavior and the header wrapper spacing assumptions:

• on mobile, the visible toggle could fail to drive the sidebar state update path that Pocodex was relying on
• the old geometry checks could still treat the sidebar as open after the shell had collapsed, which caused follow-up interactions to restore it unexpectedly
• the header wrappers started inheriting a host-side safe-header offset that is not correct in the browser shell

Impact

• mobile sidebar open/close is stable again
• tapping into the composer from the mobile overlay closes the sidebar before focus and keyboard layout changes
• typing no longer forces the sidebar back open
• header/tab icons return to the expected alignment

Validation

• pnpm run check:commit
• Playwright mobile smoke checks for sidebar close/open, focus/typing after close, and composer focus from the open overlay state
• Playwright desktop and mobile visual checks for header spacing