feat(i18n): add missing Japanese (ja) translations for the sign-in flow

[greymoth-jp]

Problem

The Japanese locale (frontend/resources/translations/ja-JP.json) is behind en-US (2776 vs 2410 keys). In particular the signIn section had 40 missing keys, so the sign-in, password-reset, and verification-code screens fell back to English for Japanese users.

Change

Adds the 40 missing signIn.* keys, completing the full authentication surface (sign-in, password reset, login-code / verification, local mode). Follows the existing Japanese house style — noun-form button labels (パスワードをリセット), 〜してください for instructions, product names kept in English (AppFlowy Cloud, Magic Link, Google, GitHub), and \n preserved verbatim. Key order mirrors en-US.json; existing translations are untouched.

Testing

• ja-JP.json validated as valid JSON; change is scoped to one file (40 insertions / 0 deletions), LF line endings.

The other missing sections (ai / document / settings, ~300+ keys) are intentionally left for follow-up PRs to keep this review small.

Summary by Sourcery

New Features:

• Add Japanese translations for all missing sign-in, password reset, and verification code screen strings in the ja-JP locale.

[CLAassistant]

All committers have signed the CLA.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Adds the 40 missing Japanese signIn.* translations in ja-JP.json so the full sign-in and authentication flow no longer falls back to English, keeping key order aligned with en-US and following the existing Japanese copy style.

File-Level Changes

Change	Details	Files
Complete Japanese translations for the sign-in and authentication flow to eliminate English fallbacks.	Added all 40 missing signIn.* keys for Japanese, covering sign-in, password reset, verification/login code, and local mode flows Matched key ordering to the en-US locale file while leaving existing Japanese translations unchanged Followed existing Japanese localization conventions for tone, button-label style, product name handling, and newline preservation Verified the updated ja-JP translations file is valid JSON with consistent line endings	frontend/resources/translations/ja-JP.json

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[github-actions]

🥷 Ninja i18n – 🛎️ Translations need to be updated

Project /project.inlang

lint rule	new reports	level	link
Identical pattern	1	warning	contribute (via Fink 🐦)

[sourcery-ai]

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[greymoth-jp]

On the Ninja i18n identicalPattern warning for signIn.switchToAppFlowyCloud: that value is the product name "AppFlowy Cloud", so the Japanese string is intentionally identical to English — JP localization keeps brand names in Latin script rather than transliterating them. The other 39 keys are fully translated. Happy to suppress it in the lint config instead if you prefer a clean check.