[client] Add Japanese (ja) UI translation (#6790)

## Describe your changes
Adds a Japanese (ja) locale for the desktop UI, following the procedure
in `client/ui/i18n/TRANSLATING.md`.

- New `client/ui/i18n/locales/ja/common.json` — all 441 keys from the
`en` source bundle, `message` only, same key order.
- Registered in `client/ui/i18n/locales/_index.json`: `{"code": "ja",
"displayName": "日本語", "englishName": "Japanese"}`.

Translation notes:
- Placeholders (`{version}`, `{count}`, `{name}`, `{remaining}`, ...),
`\n`, trailing `…`/`...`, the leading space in
`notify.update.enforcedSuffix`, and the `<docs>…</docs>` inline-link
tags are all preserved verbatim.
- Brands kept as-is: NetBird, WireGuard® (® preserved), Rosenpass,
GitHub, NetBird Cloud. Acronyms kept: SSO, DNS, IP/IPv6, ACL, SSH, JWT,
TTL, SFTP, MTU, PSK, LAN, P2P, ICE, IdP.
- Formal register (です・ます); short labels for buttons/tray; quoted UI
labels use Japanese brackets 「」.

No code changes are required: the React frontend auto-loads every
`locales/*/common.json` via `import.meta.glob`, the tray/Go side embeds
the tree via `//go:embed all:i18n/locales`, and the language picker
lists whatever `_index.json` declares.

### Verification
- `go test ./client/ui/i18n/` passes.
- JSON valid; key set and order identical to `en`; no `description`
fields; no empty messages; placeholder/`\n` counts match `en`.
- Built the Windows UI and confirmed the General, Network, Security, and
Troubleshooting settings tabs plus the main window render correctly in
Japanese (language picker shows "日本語 (Japanese)"), with no text
truncation, overflow, or leaked placeholders.

## Issue ticket number and link
N/A

## Stack

<!-- branch-stack -->

### Checklist
- [ ] Is it a bug fix
- [ ] Is a typo/documentation fix
- [x] Is a feature enhancement
- [ ] It is a refactor
- [ ] Created tests that fail without the change (if possible)
- [x] This change does **not** modify the public API, gRPC protocols,
functionality behavior, CLI / service flags, or introduce a new feature
— adding a shipped UI locale is the documented extension path in
`client/ui/i18n/TRANSLATING.md`.

> By submitting this pull request, you confirm that you have read and
agree to the terms of the [Contributor License
Agreement](https://github.com/netbirdio/netbird/blob/main/CONTRIBUTOR_LICENSE_AGREEMENT.md).

## Documentation
Select exactly one:

- [ ] I added/updated documentation for this change
- [x] Documentation is **not needed** for this change — UI translation
strings only; no user-facing product docs are affected.

### Docs PR URL (required if "docs added" is checked)
N/A

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- codesmith:footer -->
---
<a
href="https://app.blacksmith.sh/netbirdio/codesmith/netbird/pr/6790"><picture><source
media="(prefers-color-scheme: dark)"
srcset="https://pr-comments-assets.blacksmith.sh/codesmith/view-with-codesmith-dark-v2.svg"><source
media="(prefers-color-scheme: light)"
srcset="https://pr-comments-assets.blacksmith.sh/codesmith/view-with-codesmith-light-v2.svg"><img
alt="View with Codesmith"
src="https://pr-comments-assets.blacksmith.sh/codesmith/view-with-codesmith-dark-v2.svg"></picture></a>
<a
href="https://backend.blacksmith.sh/track/enable-autofix?expires=1786742881&installation_id=146802194&pr_number=6790&repository=netbirdio%2Fnetbird&return_to=https%3A%2F%2Fgithub.com%2Fnetbirdio%2Fnetbird%2Fpull%2F6790&signature=a1dcd7faf63ae10071ca2da3639b4daf6505eeb0258790e622222e742a8ebcb6"><picture><source
media="(prefers-color-scheme: dark)"
srcset="https://pr-comments-assets.blacksmith.sh/codesmith/autofix-with-codesmith-dark.svg"><source
media="(prefers-color-scheme: light)"
srcset="https://pr-comments-assets.blacksmith.sh/codesmith/autofix-with-codesmith-light.svg"><img
alt="Autofix with Codesmith"
src="https://pr-comments-assets.blacksmith.sh/codesmith/autofix-with-codesmith-dark.svg"></picture></a>
<sup>Need help on this PR? Tag <code>/codesmith</code> with what you
need. Autofix is disabled.</sup>

<!-- codesmith:autofix:disabled -->
<!-- /codesmith:footer -->

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
  * Added Japanese as a supported interface language.
* **Localization**
* Provided full Japanese translations for the UI, covering navigation,
settings, onboarding, desktop/tray notifications, connection and status
messaging, troubleshooting, update/about screens, and
authentication/error dialogs.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>
Co-authored-by: Eduard Gert <kontakt@eduardgert.de>

client/ui/i18n/locales/_index.json

@@ -8,6 +8,7 @@
         {"code": "fr", "displayName": "Français", "englishName": "French"},
         {"code": "it", "displayName": "Italiano", "englishName": "Italian"},
         {"code": "pt", "displayName": "Português", "englishName": "Portuguese"},
-        {"code": "zh-CN", "displayName": "简体中文", "englishName": "Simplified Chinese"}
+        {"code": "zh-CN", "displayName": "简体中文", "englishName": "Simplified Chinese"},
+        {"code": "ja", "displayName": "日本語", "englishName": "Japanese"}
     ]
 }