feat: add dynamic model effort selection

[xiang-lee]

Summary

• add a dynamic effort selector next to the model picker when the selected model exposes variants
• persist the selected variant per session and send it through prompt_async
• hide the selector for models that do not advertise any variants

Changes

• decode model variants from /config/providers
• restore and persist selectedVariantBySession
• normalize invalid saved variants when switching models or sessions
• include variant in prompt requests and message decoding
• add a chat toolbar menu for Auto / None / Low / Medium / High / Extra High based on the currently selected model

Why

The server already exposes model-specific effort/variant capabilities, but the iOS client only allowed model selection. This change keeps the UI driven by server config instead of hardcoding per-model options, so the same client works with providers that expose different variant sets.

Validation

• xcodebuild -project OpenCodeClient/OpenCodeClient.xcodeproj -scheme OpenCodeClient -configuration Debug -destination 'generic/platform=iOS Simulator' build
• manually verified against a running opencode server that exposes variants for GPT-5.4
• confirmed the effort menu only appears for models with variants and that Extra High can be selected

Screenshot