Improve error semantics for model fetch requests

[Lakr233]

Summary

• handle error, response type, and non-2xx status for hub model search request
• add optional error callbacks while keeping existing completion call sites compatible
• distinguish endpoint/model/request/json/empty-body failures in cloud model list fetching

Scope

• FlowDown/Interface/SettingController/Model/ModelDownloadController/HubModelDownloadController+RemoteModel.swift
• FlowDown/Backend/Model/ModelManager+Cloud.swift

Verification

• rely on GitHub Actions for build and tests

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	flowdown-landing	6c9c43d	Commit Preview URL Branch Preview URL	Feb 20 2026, 12:33 PM