[AFK] Surface local server and save failures in the desktop app

[liangming99]

What to build

Handle common local failure states in the desktop workflow so users get clear recovery options instead of a blank window or failed save. Cover port conflicts, missing files, invalid selections, and write failures.

Acceptance criteria

• If the preferred local port is unavailable, the desktop app either chooses another safe port or shows a clear recovery message.
• If the selected file is missing, not an HTML file, or cannot be written, the app shows an actionable error and does not start a broken editing session.
• Save failures from the local save endpoint are surfaced in the desktop UI or editor status area.

Blocked by

• [AFK] Enable autosaving edits from the Electron preview #4

Triage

Type: AFK
Intended label: ready-for-agent

[liangming99]

Created in error by an automation run. Closing immediately; development planning is being moved to local markdown files and should not be tracked in this upstream repository.