Persist last selected model when opening new threads

[WeldFire]

What Changed

When a user selects a model in any thread and then opens a new thread, the new thread now pre-populates with that last selected model instead of resetting to the project/global default.

• Added lastSelectedModel and lastSelectedProvider as persisted global fields in composerDraftStore
• setModel and setProvider now write these globals whenever a non-null value is explicitly selected
• useHandleNewThread reads these globals and pre-populates fresh thread drafts before navigating

Existing threads are unaffected as the new values are only written to freshly generated thread IDs that have no prior draft state. Started threads have their model locked via lockedProvider/sessionProvider in ChatView regardless.

Why

Model selection was scoped to individual thread drafts (draftsByThreadId[threadId].model). When a new thread was created its draft started with model: null, causing selectedModel in ChatView to fall back to activeProject.model ?? getDefaultModel() silently discarding the user's recent choice. This fix introduces a lightweight "sticky last selection" that survives navigation and page reloads via localStorage, without requiring a new user-facing setting.

UI Changes

No UI changes. The model picker component is unchanged, it just displays the pre-populated value on new threads.

Demo

• This demo shows GPT-5.4 selected initially
• A second thread is opened showing GPT-5.4 again (behavior unchanged)
• The second thread chat model is changed to GPT-5.2 and a prompt is sent (behavior unchanged)
• A new third thread is opened and shows GPT-5.2 selected automatically (new behavior)
• The original thread is opened showing that GPT-5.4 is unchanged (behavior unchanged)

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

Note

Persist last selected model and provider when opening a new thread

• Adds lastSelectedModel and lastSelectedProvider fields to the composer draft store, persisted alongside thread drafts.
• When setModel or setProvider is called with a non-null value, the store records it as the last selection; null values leave the persisted selection unchanged.
• In useHandleNewThread, newly created threads are initialized with the last non-null model and provider from the store.
• On rehydration, persisted values are normalized via normalizeModelSlug and normalizeProviderKind before being merged into live state.

^{Macroscope summarized 2813e5a.}

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[ianjamesburke]

Just spent 10 minutes writing this feature request lol. nice work

what do you think about add a setting in the model card for setting the default model when creating new chats.

With a drop down containing "last used" and a list of providers/models