#3859 fix(desktop): give v2 chat pane full width

Summary

• v2 ChatPaneInterface root sat inside PaneContent's row-direction flex with no width directive, so it sized to its intrinsic content and the chat appeared smushed. Added w-full to match every other v2 pane (TerminalPane, FilePane, etc.).
• Restored w-full on the v2 SessionSelector trigger button — present in v1 but dropped in the port.

Test plan

• Open a v2 workspace chat pane and confirm the message list, composer, and session selector fill the pane width
• Resize/split the pane and confirm content reflows instead of clipping

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Summary by cubic

Fixes the v2 chat pane width so the message list, composer, and session selector span the full pane. Prevents the smushed layout and clipping when panes are split or resized.

• Bug Fixes
  • Add w-full to the ChatPaneInterface root inside the row-direction flex.
  • Restore w-full on the SessionSelector trigger button.

Summary by CodeRabbit

• Style
  • Improved chat interface layout and width handling for better visual presentation and alignment.