chore(ai): gate verbose AI logging to avoid emitting user data in release builds

[HankYuLinksys]

Description

The AI Assistant logs a large amount of diagnostic detail unconditionally, including router state and conversation content. This is useful during development but should not run in production builds.

Current behaviour

lib/ai/orchestrator/router_chat_controller.dart:

• Logs the full router context on construction: _log('Router context:\n$routerContext')
• Logs assistant message previews: textContent.substring(0, ...clamp(0, 200))
• Logs user message content verbatim

lib/ai/providers/usp_command_provider.dart:

• _getConnectedDevices() iterates every device and logs name, IP, connection type and signal strength
• Mesh extender names and MAC addresses are logged
• buildRouterContext() logs model name, firmware version, WAN IP and per-band SSIDs

generative_ui's AwsContentGenerator additionally debugPrints the request body preview (up to 500 chars), which includes the system prompt and conversation.

Why this matters

Device names, IP/MAC addresses, SSIDs and conversation content are user data. Logging them unconditionally is a privacy concern in shipped builds.

Proposed approach

• Gate the verbose [AI] logging behind kDebugMode (or an existing project-level log-level mechanism if one is preferred — worth checking what the logger already supports)
• Keep structural / lifecycle logs (loop iteration, tool name, success/failure, token counts) since they are useful and non-sensitive
• Drop or redact: full router context dump, message content previews, per-device identifiers
• The debugPrint calls inside generative_ui need a separate change in the UI kit repo — track separately if it turns out to be needed

Acceptance criteria

• Verbose content logging is disabled in release builds
• Device identifiers (IP / MAC / hostname) and SSIDs are not logged unconditionally
• Conversation content is not logged unconditionally
• Non-sensitive diagnostic logs (loop progress, tool names, token usage) are retained
• flutter analyze clean

Context

Follow-up to #929.