feat(locales): add french locale, fix missing error keys, remove dead keys

Adds french locale registration and translation file. Fixes 4 missing error keys that were referenced in code but absent from all locale files. Removes 2 unreferenced keys from `register.validation` that duplicated `registerPasskey.errors`.

I *think* that's all that's needed for french locale? Couldn't find anything else required to make it work!

frontend/src/lib/i18n.ts

@@ -2,7 +2,7 @@ import { _, getLocaleFromNavigator, init, locale, register } from "svelte-i18n";
 
 const LOCALE_STORAGE_KEY = "tranquil-pds-locale";
 
-const SUPPORTED_LOCALES = ["en", "zh", "ja", "ko", "sv", "fi"] as const;
+const SUPPORTED_LOCALES = ["en", "zh", "ja", "ko", "sv", "fi", "fr"] as const;
 export type SupportedLocale = typeof SUPPORTED_LOCALES[number];
 
 export const localeNames: Record<SupportedLocale, string> = {
@@ -12,6 +12,7 @@ export const localeNames: Record<SupportedLocale, string> = {
   ko: "한국어",
   sv: "Svenska",
   fi: "Suomi",
+  fr: "Français",
 };
 
 register("en", () => import("../locales/en.json"));
@@ -20,6 +21,7 @@ register("ja", () => import("../locales/ja.json"));
 register("ko", () => import("../locales/ko.json"));
 register("sv", () => import("../locales/sv.json"));
 register("fi", () => import("../locales/fi.json"));
+register("fr", () => import("../locales/fr.json"));
 
 function getInitialLocale(): string {
   const stored = localStorage.getItem(LOCALE_STORAGE_KEY);

frontend/src/locales/en.json

@@ -359,6 +359,7 @@
     "passwordRemoved": "Password removed. Your account is now passkey-only.",
     "failedToRemovePassword": "Failed to remove password",
     "failedToLoadTotpStatus": "Failed to load TOTP status",
+    "failedToRegenerateBackupCodes": "Failed to regenerate backup codes",
     "totpEnabledSuccess": "Two-factor authentication enabled successfully",
     "totpDisabledSuccess": "Two-factor authentication disabled",
     "backupCodesCopied": "Backup codes copied to clipboard",
@@ -490,6 +491,8 @@
     "invitesEnabled": "User invites enabled",
     "invitesDisabled": "User invites disabled",
     "userDeleted": "User account deleted",
+    "failedToDeleteAccount": "Failed to delete account",
+    "failedToSaveConfig": "Failed to save server configuration",
     "failedToLoadConfig": "Failed to load server configuration",
     "signalIntegration": "Signal",
     "signalLinked": "Linked",
@@ -597,7 +600,8 @@
     },
     "error": {
       "title": "Authorization failed",
-      "tryAgain": "Retry"
+      "tryAgain": "Retry",
+      "genericError": "An unexpected error occurred"
     }
   },
   "sso_register": {