Translate core UI to Japanese

[crossn]

Summary

Adds Japanese translations for the main RMG-K UI based on the current master.

This includes:

• Main window and common actions
• Settings dialog
• Unified controller settings
• Common Qt message boxes
• The initial no-ROM empty state shown to new users

This is intended as the first core UI translation pass for the upcoming release. Some lower-frequency/debug strings and plugin-specific UI remain for follow-up work.

Validation

• 480 translated messages, 0 unfinished
• No duplicate translation keys
• No placeholder or mnemonic mismatches
• Unified controller settings coverage: 147/147
• Clean Windows Release build using MSYS2 UCRT64
• lrelease: 480 finished, 0 unfinished
• Manual GUI smoke test completed for:
  • Main window
  • Initial no-ROM screen
  • Settings
  • Unified controller settings
  • Application startup/shutdown

Scope

This PR intentionally focuses on the core UI needed for the upcoming release. Remaining low-frequency/debug strings and plugin-specific UI can be handled separately.