Fix: Preserve user's response language (#7)

[Subharup-31]

Summary

• Updates Clicky's companion response prompt to answer in the same natural language as the user's latest message.
• Explicitly covers Russian and other non-English inputs so responses do not randomly switch to Spanish, Italian, or English.
• Keeps translation and language switching available when the user explicitly asks for it.

Problem

Issue #7 reports that Clicky can reply in the wrong language when the user speaks or writes in Russian or another non-English language. The response prompt did not directly instruct the model to preserve the user's current language, so multilingual interactions could drift into unrelated languages.

Fix

Added a focused language-preservation rule to the companion voice response system prompt in CompanionManager.swift. The rule tells Clicky to respond in the same natural language as the user's latest message, while still allowing explicit translation or language-switch requests.

Verification

• Ran swiftc -parse leanring-buddy/*.swift successfully.
• Reviewed the prompt flow to confirm the new instruction is included in every companion voice response request.
• Tested locally and confirmed the fix works as expected.

Closes #7