Let the Thinking section be collapsed while the model is still thinking

[salmanmkc]

Fixes #534.

The Thinking expander was forcing isExpanded = true on every recomposition while inProgress was true, so tapping the header to collapse it had no visible effect until the model stopped thinking. Initialize the state from inProgress once with remember instead, the section still auto-opens when the thinking message first appears, but the user can collapse it whenever.

Test plan

• ./gradlew :app:compileDebugKotlin passes.
• Logic check: first composition runs with inProgress = true, so remember { mutableStateOf(inProgress) } starts expanded. Subsequent recompositions don't re-init the state, so user taps stick.

Tested like this:

This is with the fix (Verified locally on an emulator using a temporary debug injection that adds a ChatMessageThinking(inProgress = true) on chat open (not part of this PR, used only for repro). With the fix applied, tapping the "Show thinking" header while inProgress is still true correctly collapses the section. Without the fix, the tap had no effect until streaming finished.)

ChatMessageThinking(
        content = "Hmm, let me think about this. The user is asking about a copy button...",
        inProgress = true,
      ),

Before the fix, the behavior is broken, we couldn't collapse:

CleanShot.2026-05-12.at.03.40.44.mp4

After the fix, now we can collapse:

CleanShot.2026-05-12.at.03.35.13.mp4