v0.21.0 — rearrangeable workspace with savable presets

Rearrange the workspace and save named layouts.

The document window's three panes (sidebar / center / inspector) are now resizable and collapsible per editor mode, and any arrangement can be saved as a named workspace preset.

• Resizable / collapsible panes — drag the inspector divider, ⌥⌘I to hide/show the inspector, resize or collapse the sidebar. Each of Cue / Lyric / Show remembers its own arrangement, and the current layout survives relaunch.
• Named workspace presets — View ▸ Workspace: Save Current Layout As…, Overwrite, Manage Workspaces… (rename / delete), Reset to Default. A built-in Default can't be deleted or overwritten.
• Snapshot, not a live binding — dragging a divider never silently rewrites the selected preset. Applying a preset affects only the frontmost window, and clamps to fit a narrow window without altering the stored preset.
• No document-format change — layouts live in app preferences, not the .cuelist.

Implements #714 (phase A). Builds on v0.20.0.

Notes: the app is ad-hoc signed (free-tier) — on first launch, right-click → Open, or run xattr -dr com.apple.quarantine /Applications/OnlyCue.app. This release's UI/end-to-end tests (including the window min-width guard) were verified by code review; the on-device UI-automation run is pending a runner-environment fix.