Add favorite models and pricing to model selector

[GSmithBoltz]

What

Two related improvements to the model selection sheet (used by both the chat top bar and the Settings default-model picker):

1. Favorite models — each model row now has a star toggle. Favorited models are pinned to the top of the list (and stay on top within search results). Favorites are persisted in MMKV (favorite_models key) and exposed as a Flow from ModelRepository, so both screens stay in sync live — same pattern as the font-size settings.

2. Per-model pricing — the sheet now shows OpenRouter input/output token cost on each row, e.g. deepseek-v4-pro $0.44/M in | $0.87/M out. Free models show Free; models with dynamic pricing (e.g. openrouter/auto, which reports -1) show nothing. To get pricing, getModels() now fetches /api/v1/models directly with ktor and parses it with kotlinx.serialization instead of going through the openai-kotlin client, which drops the pricing field. Side benefit: the models list loads even before an API key is set, since the endpoint is public.

Implementation notes

• New dependency: ktor-client-core in :data (version managed by the existing openai-client BOM, same as the already-present ktor-client-okhttp).
• Model/ModelEntity gain nullable inputPricePerMillion/outputPricePerMillion fields with defaults, so previously stored default-model JSON still deserializes fine.
• UI uses only basics components (IconButton, Icon, Text), mirroring the star pattern from ChatOptions.
• No migrations, no new permissions.

Testing

• :app:compileDebugKotlin, testDebugUnitTest, and :app:assembleDebug all pass.
• ktlintCheck passes for :domain, :data, and :app. The :ui check reports only violations that already exist on master (Slider.kt, MessageBubble.kt, SettingsScreen.kt import order) — this change adds none.
• Manually tested on a physical device: starring/unstarring, pinning order, search interaction, persistence across restart, and pricing display in both the chat sheet and Settings.

Screenshot

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