fix(chat/effort): drive caret icon by dropdown state

[LoneRifle]

Problem

The current chat window is reliant on CarbonCaretDown, which may not be the most suitable depending on context.

In particular:

• the caret next to the model display name is slightly misleading, given that it opens the settings modal
• the caret used in ThinkingEffortChip could change depending on whether the dropdown menu is open

Solution

Replace CarbonCaretDown with more suitable icons

• use a gear icon next to the model display name so it is clearer that tapping would bring up the settings modal submitted separately as fix(chat): use lucide/settings-2 icon for settings #2300, using lucide/settings-2
• listen to thinking effort dropdown menu state, and use that to determine whether caret down (closed) or caret up (open) should be displayed

Screenshots

Before

After

[gary149]

idk this feels a bit weird to me tbh

[LoneRifle]

idk this feels a bit weird to me tbh

I've reworked the PR so that it adds just the effort dropdown for now. Choice of icon for model id indicator resubmitted as #2300