Add user-facing error notifications via NetBeans status bar and dialogs

[sfloess]

Part of #27: Improve Error Handling and Logging

Scope

Add user-friendly error notifications for AI interaction failures using NetBeans platform UI mechanisms (status bar, notification bubbles, or dialogs).

What to do

• Identify all error paths that currently fail silently or only log to console
• Add StatusDisplayer.getDefault().setStatusText() for transient errors
• Add NotifyDescriptor dialogs for critical errors requiring user action (e.g., invalid API key, network unreachable)
• Provide actionable messages: "API key not configured. Open Settings > AI to add your key."
• Distinguish between retryable errors (show retry option) and permanent errors (show config guidance)

Acceptance criteria

• Network errors show a user-visible notification (not just a log entry)
• API auth failures direct the user to settings
• Rate limit errors show appropriate guidance (wait or upgrade plan)
• No error fails completely silently from the user's perspective