fix: Make diff wrapping toggle always available in command_list and correct a type error (resolves #10682)

[ariane-emory]

What does this PR do?

• Move diff wrapping toggle from session-level to global app-level commands
• Corrects a type error by replacing local diffWrapMode signal with kv.signal('diff_wrap_mode', 'word')
• Follow pattern from issue fix(tui): Move animations toggle to global System category (resolves #10495) #10497 fix for animations toggle
• Toggle now accessible before sending first message

How did you verify your code works?

Manual testing and bun typecheck

Resolves #10682.

[github-actions]

Hey! Your PR title Make diff wrapping toggle always available in command_list (resolves #10682) doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

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