feat(app): add /thinking command to toggle reasoning visibility

[hotic]

What does this PR do?

Adds /thinking slash command to WebUI, matching TUI behavior for toggling thinking/reasoning block visibility.

• Default: visible (same as TUI)
• Command toggles show/hide state

Fixes #7866

How did you verify your code works?

Tested locally with bun run --cwd packages/app dev:

• /thinking appears in slash command autocomplete
• Executing toggles visibility of reasoning blocks
• Behavior matches TUI /thinking command