Make companion window freely resizable with persisted bounds

[LordVicky]

Summary

• Main window is now freely resizable and maximizable instead of hard-locked to 1120×630 × UI scale; minimum size still tracks the UI Scale setting so the layout never squeezes below its designed size
• UI Scale changes now only raise the window minimum (growing it if needed) and never override a user-chosen size; scale keeps controlling zoom exactly as before
• Window bounds (position, size, maximized) persist to window-state.json in userData, restored clamped on-screen at launch
• showWindowCentered → showMainWindow: only recenters when the window isn't visible on any display, so tray/show no longer stomps the saved position
• 4px no-drag strip keeps the top resize edge usable on the frameless titlebar
• New pure module window-state.ts with unit tests for clamp/restore/persistence edge cases

Testing

• 333 companion tests pass (14 new), tsc clean
• Hardware-verified on Hyprland/Wayland: edge/corner resize, maximize, state saving (note for testers: quit any running installed instance first — the single-instance lock otherwise shows the old window)

Spec: docs/superpowers/specs/2026-07-09-resizable-window-design.md
Plan: docs/superpowers/plans/2026-07-09-resizable-window.md

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