fix(adopt): fail clearly when Legacy Desktop adoption can't source ComfyUI

[Kosinkadink]

Summary

When in-place adoption of a Legacy Desktop install could not obtain the ComfyUI source (no pre-swap staged copy and the git clone failed), the source-missing dialog offered a "Switch to managed env" button. Choosing it threw source-missing-switch-to-managed, which the migrate dispatcher mapped to { ok: true, navigate: 'new-install' }.

But navigate: 'new-install' is unhandled in the renderer (only 'list'/'detail' are), so the operation reported success while doing nothing — leaving the user back where they started (the legacy detail view on the dashboard-tile path, or the chooser on the first-time-migration path). The button label was also misleading: it sounded like a lightweight env swap but really pointed at a fresh install that abandons the legacy data and launch args.

Fix

Replace the fake-success escape with a clear failure (per #917):

• Drop the "Switch to managed env" option from the source-missing prompt; keep Retry (transient network/git failures) and Cancel.
• On any non-retry choice, throw a source-missing: error that the dispatcher surfaces as { ok: false, message } with a user-friendly message that suggests doing a new install.
• This fixes both the dashboard-tile and first-time-migration surfaces at once, since both flow through the same backend + ProgressModal error banner.

Also fixes a latent bug: an unexpected prompt choice previously break-ed the source loop silently, leaving sourceMode null and adoption continuing without a source.

Changes

• desktopAdopt.ts — drop 'switch-to-managed' from UserChoice; source loop throws a clear error on anything but explicit retry.
• migrate.ts — remove the "Switch to managed env" button and the fake-success remap; source failures return { ok: false, message } with a friendly message.
• locales/en.json — remove adoptPromptSwitchToManaged, clarify the prompt message, add adoptSourceMissingFailed.
• errorBucket.ts — update stale comment (source_missing bucketing still works).
• migrate.test.ts — assert fail-clearly behavior.

Verification

• pnpm typecheck / lint / build pass.
• migrate.test.ts, desktopAdopt.test.ts, telemetry.test.ts pass.

Closes #917

[Kosinkadink]

Manual testing still pending

Automated checks pass (typecheck, lint, build, and the migrate / desktopAdopt unit tests), but the Legacy Desktop adoption happy path has not yet been verified end‑to‑end on a real machine. Recording what's left so it isn't missed before merge.

What still needs a manual run

1. Dashboard tile → adopt (happy path). From the dashboard, click the ComfyUI Legacy Desktop tile and confirm it now runs the adopt flow cleanly — no "Switch to managed env" option, and a new adopted install is registered on success.
2. Clear failure + retry on source-missing. Force the source‑missing condition (no staged copy + clone unavailable) and confirm the user now gets a clear error with a Retry option, and that cancelling does not silently fall through to a fake "new install" success.
3. No user data loss on failure. Confirm that an aborted/failed adoption removes only the new adopted install dir under ComfyUI-Installs\ and never touches the legacy base dir (user/, input/, output/).
4. Startup‑arg porting. Confirm the adopted install's launchArgs reflect the legacy Desktop settings. With an empty legacy Comfy.Server.LaunchArgs the synthesized default should be --listen 127.0.0.1 --port 8000 --enable-manager. A richer check (setting a server‑config UI toggle in legacy Desktop first, then confirming it carries into launchArgs) is still worth doing.

Already confirmed by code inspection

• Dashboard tile for sourceId === 'desktop' routes to adoptDesktopInstall (adoption is the migration method, not standalone snapshot migration).
• On failure, migrate.ts removes the adopted record + rms the adopted installPath only — legacy base dir is untouched.
• The old fallback (source-missing → fake { ok: true, navigate: 'new-install' }) was a no‑op that the renderer ignored; this PR replaces it with a clear failure message.

A local test environment has been staged (clean fresh‑adopt state, marker cleared, full backup taken); just need to drive the UI.