Complete Japanese translations

[caydyan]

Summary

• Completes missing Japanese strings in chat/contact controls and onboarding resources.
• Replaces remaining user-visible English fallbacks in the touched Japanese resource files.
• Adds the missing Firebase notification values-ja resource file.

Fixes #459.

Validation

• Parsed every sphinx/**/src/main/res/values/strings.xml and matching values-ja/strings.xml with Python XML parsing.
• Verified every translatable base key has a Japanese resource counterpart and no unexpected English-identical Japanese strings remain, excluding intentional brand/protocol labels like Sphinx FCM, sat, sats, http, and https.
• git diff --check
• ./gradlew :sphinx:screens:chats:chat-common:chat-common:mergeDebugResources :sphinx:screens:onboard:onboard-connect:mergeDebugResources :sphinx:screens:onboard:onboard-resources:mergeDebugResources :sphinx:screens-detail:chat-detail:tribe-detail:mergeDebugResources --no-daemon --stacktrace passed under JDK 17.

Note: Firebase notification resource merge was separately attempted with Google Services processing excluded, but local validation is blocked by dependency downloads from dl.google.com terminating TLS handshakes for Firebase/Play Services AARs.

Bounty payout address: bc1qev5ant33v5y89qqjvcf4mh9hlax5svqf5xd7gc

[caydyan]

Current-head validation rerun on c4819fb9acd122386b034fdf218bca774a30e390:

• Structured XML/key coverage passed for all 5 touched values-ja/strings.xml files against their base values/strings.xml files.
• git diff --check origin/master...HEAD passed.
• ./gradlew :sphinx:screens-detail:chat-detail:tribe-detail:mergeDebugResources :sphinx:screens:chats:chat-common:chat-common:mergeDebugResources :sphinx:screens:onboard:onboard-connect:mergeDebugResources :sphinx:screens:onboard:onboard-resources:mergeDebugResources passed.

I also attempted ./gradlew :sphinx:service:features:notifications:feature-service-notification-firebase:mergeDebugResources; it reaches processDebugGoogleServices and is blocked locally because this checkout does not include google-services.json. No Japanese resource XML/key issue was reported before that environment/config gate.

Local worktree is clean after the rerun.