i18n(ja_JP): complete Japanese translation for all untranslated strings

[kazaminosuke]

Summary

• frontend/src/assets/locales/ja_JP.json — Translated all remaining English strings to Japanese
• build/windows/inno/project.iss — Added Japanese language support to the Inno Setup installer

Changes

ja_JP.json

Translated all previously untranslated (English) keys across the following sections:

Section	Keys updated
settings.tabs.privacy	privacy label
settings.experimental	Title, description, instance backup warning dialog (all body/confirm keys)
settings.privacy	Microsoft Clarity analytics description, link labels
versions.edit.backup	experimental_disabled hint
mods	action_update, action_uninstall, batch_update_*, batch_uninstall_*, update_available, update_latest, no_update_source, action_result_*
errors	ERR_LL_NOT_SUPPORTED, ERR_FETCH_LL_DB, ERR_INVALID_VERSION_FORMAT, ERR_LIP_INSTALL_FAILED, ERR_LIP_PACKAGE_*, ERR_LIP_CACHE_CLEAN_FAILED, ERR_LL_MANAGED_IN_VERSION_SETTINGS
lip	ll_version_label/placeholder, game_version_*, files.* (all), sort_options, order_options, task_console.*, title, by_author_inline, package_details_aria_label
curseforge	mod_details_aria_label, loading_description, mod_tabs.*, mod_files.columns.*
clarity.prompt	All keys (title, body, enable, disable, link labels)

build/windows/inno/project.iss

• Added Japanese.isl (bundled with Inno Setup) to the [Languages] section
• Added japanese.* entries for all 15 [CustomMessages] keys:
  • WebView2 runtime installation status message
  • Close-before-uninstall prompt
  • Optional data cleanup dialog (title, description, warning, checkbox labels, folder-not-found suffix)
  • Upgrade mode and NSIS migration mode detection messages

Test plan

• Set language to ja_JP and verify all updated screens display Japanese text
• Check Settings > Privacy tab label and content
• Check Settings > Experimental > Instance Backup warning dialog
• Check Mods page batch update / uninstall labels and result dialogs
• Check lip page filters, file list, and task console output
• Check CurseForge mod details panel and Files tab columns
• Run the installer, select Japanese, and verify all dialogs render in Japanese