Localize theme command feedback

[w287346141]

Summary:

• Route /theme success and unknown-theme feedback through i18n.
• Add Simplified Chinese copy for saved and unknown theme messages.
• Cover EN compatibility and zh-CN feedback in slash command tests.

Verification:

• npm.cmd run test -- slash.test.ts
• npm.cmd run typecheck
• npm.cmd run verify

[w287346141]

Superseded by #13. I folded the localized /theme command feedback from this PR into #13 so the maintainer can merge one theme-related PR without resolving the #12/#13 conflict manually.