Adds a chevron-right collapse button after the last tab (Files, Git,
Terminal, Info) in the workspace panel header. Clicking it hides the
workspace panel and lets the main chat column fill the full width.
When collapsed, an expand strip with a chevron-left button appears
below the context bar to re-open the workspace panel.
Changes:
- Add workspacePanelCollapsed boolean to AppState
- Add collapse button to WorkspacePanel header tabs
- Add expand strip with toggle button in main column when collapsed
- Adjust grid layout to hide panel and stretch main when collapsed
- Handle collapsed state at ≤1180px responsive breakpoint
