🤖 refactor: persist model settings to backend instead of localStorage

[ammar-agent]

Currently defaultModel, hiddenModels, and compactionModel (via modeAiDefaults) are stored in localStorage. This creates issues:

• Settings don't sync across devices
• Settings are lost if browser storage is cleared
• Inconsistent with other config that uses the backend (~/.mux/config.json)

Current state

• defaultModel: usePersistedState('model-default', ...)
• hiddenModels: usePersistedState('hidden-models', ...)
• compactionModel: usePersistedState(MODE_AI_DEFAULTS_KEY, ...) → modeAiDefaults.compact.modelString

Proposed change

Move these to backend config and expose via api.config methods. The backend already has modeAiDefaults support—extend it or add dedicated fields.

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Generated with mux

[ethanndickson]

Enables #1208