Add optional per-desktop layouts with live desktop switching

[dartcafe]

What do you think? Valuable?

This feature allows optional per desktop layouts, so you can have different layouts per desktop and display.
If not set, it falls back to the default layout of the particular display or the built in default.

Summary

• New enablePerDesktopLayouts setting (default off, backward-compatible):
  once enabled, each virtual desktop gets its own independent layout,
  cloned from the display's shared layout the first time that desktop is
  edited — it diverges from then on instead of aliasing the same shared
  object as other not-yet-customised desktops.
• The layout editor now stays open across a desktop switch: hover the
  mouse at the left/right screen edge to reveal an arrow that switches to
  the adjacent desktop, or use your own workspace hotkey — either way the
  editor immediately reflects the newly active desktop's layout. Closing
  the editor restores whichever desktop was active when it was opened.

Fixes found while building this

• extension.js: uuid was hardcoded instead of read from the meta
  argument Cinnamon passes to init(). Now reads it correctly, which also
  makes running a differently-named fork side by side with the original a
  non-issue (separate config directories).
• grid-editor.js: the load/save preset dialogs are separate top-level
  actors drawn on top of the main canvas, but motion/button/key events
  still reached the editor operations underneath while a dialog was open
  (e.g. silently starting a split preview behind the dialog). Now guarded.
• grid-editor.js: preset preview tiles applied their inset margin twice
  (once in calculateRects, again in the drawLayout call), which
  cancelled out and left the grid preview flush against the tile edge
  instead of centered. Fixed.

[dartcafe]

Addressing some Copilot findings:

• ce2f2ee fixes too late dialog guard
• ada8f84 changes the log level for missing layout files to reduce error logging noise