Remove ui-desks interface

[shaunandrews]

Proposed Changes

• Removes the experimental ui-desks interface from Studio so the app no longer exposes or carries the alternate desk/canvas experience.
• Keeps the supported Studio renderer paths intact: default Studio continues to use the legacy renderer, while the agentic UI flag uses the maintained @studio/ui renderer.
• Cleans up now-unused desk state so existing app configs stop carrying obsolete desk data after migration.
• Drops the desk-only dependency footprint, including tldraw, along with stale UI-mode flags and user-facing desk wording.

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Most of this PR is deletion. The main new logic is the 06-remove-desks-config migration, which removes obsolete top-level desks data from the Desktop app config after the ui-desks interface is gone.

The migration:

• Checks whether app.json exists and has an own top-level desks property.
• Treats missing, invalid, or non-object config as "nothing to migrate".
• Locks app config with lockAppdata() before writing.
• Re-reads the config while locked, removes only desks, and preserves the rest of the file.
• Always calls unlockAppdata() in finally, including write failures.

The focused tests cover:

• needsToRun() returning true only when desks exists.
• Missing or invalid config safely skipping the migration.
• Removing only desks while preserving unrelated app config.
• Unlocking app data if the write fails.

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Do we want the migration at all?

Keeping it makes the removal tidy by deleting orphaned ui-desks state from app.json. Removing it would make this PR closer to a pure code deletion, and may be reasonable because only a small number of people likely used ui-desks and any saved desk data is unlikely to be worth preserving or explicitly cleaning up. The stale desks key would be inert once the interface and IPC/storage paths are gone.