Optimize expanded image dialog rerenders

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What Changed

• Removed the derived preview-state sync effect from ExpandedImageDialog.
• Kept only local image index state in the dialog and keyed dialog mounts from the selected preview.
• Moved the keyboard listener effect into the named useExpandedImageDialogKeyboardNavigation hook.
• Added React Scan recordings for the same preview-switch interaction:
  • Before: docs/performance/expanded-image-dialog-react-scan-before.webm
  • After: docs/performance/expanded-image-dialog-react-scan-after.webm

Why

react-doctor flagged ExpandedImageDialog for derived state in an effect, which caused an extra render after the parent selected a different preview. Deriving the preview from props and keeping only local navigation state removes that effect-driven rerender while preserving arrow-key/button navigation.

react-doctor result for touched component:

• Before: src/components/chat/ExpandedImageDialog.tsx:no-derived-state-effect
• After: cleared

UI Changes

No intended visual change. The committed before/after React Scan recordings above show the preview switch interaction with React Scan enabled.

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

Validation:

• bun fmt
• bun lint
• bun typecheck
• bunx --bun react-doctor --json --full --offline --fail-on none /workspace

  

Note

Reduce rerenders in ExpandedImageDialog by tracking image index instead of full preview state

• Replaces full preview prop mirroring in ExpandedImageDialog with a single imageIndex state, clamped via a new clampImageIndex helper to handle out-of-range or zero-count cases.
• Extracts keyboard navigation (Escape, ArrowLeft, ArrowRight) into a useExpandedImageDialogKeyboardNavigation hook in ExpandedImageDialog.tsx.
• Adds getExpandedImagePreviewKey in ExpandedImagePreview.tsx and uses it as a React key in ChatView.tsx, so the dialog unmounts/remounts on image selection changes instead of syncing via an effect.
• Behavioral Change: the dialog now resets fully (unmounts) when switching images rather than updating in place, and initial out-of-range indices are clamped to a valid position.

^{Macroscope summarized d9380cd.}