Min width = 1 column, add header minimize, Claude → PowerShell

- Width: drop the header-fits-too floor; popup width is now
  max(COLUMN_WIDTH, columns-required, tabs-required). The brand title in
  the header is allowed to ellipsize at the 216px minimum (full name
  preserved in tray hover and h1 title attr).
- Minimize: new -/▢ button in the header next to pin. Toggling hides the
  tab strip + body, locks width at COLUMN_WIDTH, and lets the existing
  ResizeObserver collapse height to just the header. State persists in
  config.minimized so reopening the popup remembers it. Add-column +
  button is hidden while minimized.
- Claude launcher: now spawns powershell.exe -NoExit -Command claude in
  the wt new-tab (was cmd.exe /K claude). Cmd-fallback updated to also
  start powershell directly when the trailing args target it.

electron/preload.js

@@ -40,6 +40,7 @@ contextBridge.exposeInMainWorld('bookmarks', {
 
   // Window
   setPinned: (value) => ipcRenderer.invoke('set-pinned', value),
+  setMinimized: (value) => ipcRenderer.invoke('set-minimized', value),
   setPopupHeight: (height) => ipcRenderer.invoke('set-popup-height', height),
   setPopupWidth: (width) => ipcRenderer.invoke('set-popup-width', width),