[docs] Add Claude command for adding missing i18n strings (#4023)

Author: christian-byrne

.claude/commands/add-missing-i18n.md

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+# Add Missing i18n Translations
+
+## Task: Add English translations for all new localized strings
+
+### Step 1: Identify new translation keys
+Find all translation keys that were added in the current branch's changes. These keys appear as arguments to translation functions: `t()`, `st()`, `$t()`, or similar i18n functions.
+
+### Step 2: Add translations to English locale file
+For each new translation key found, add the corresponding English text to the file `src/locales/en/main.json`.
+
+### Key-to-JSON mapping rules:
+- Translation keys use dot notation to represent nested JSON structure
+- Convert dot notation to nested JSON objects when adding to the locale file
+- Example: The key `g.user.name` maps to:
+  ```json
+  {
+    "g": {
+      "user": {
+        "name": "User Name"
+      }
+    }
+  }
+  ```
+
+### Important notes:
+1. **Only modify the English locale file** (`src/locales/en/main.json`)
+2. **Do not modify other locale files** - translations for other languages are automatically generated by the `i18n.yaml` workflow
+3. **Exception for manual translations**: Only add translations to non-English locale files if:
+   - You have specific domain knowledge that would produce a more accurate translation than the automated system
+   - The automated translation would likely be incorrect due to technical terminology or context-specific meaning
+
+### Example workflow:
+1. If you added `t('settings.advanced.enable')` in a Vue component
+2. Add to `src/locales/en/main.json`:
+   ```json
+   {
+     "settings": {
+       "advanced": {
+         "enable": "Enable advanced settings"
+       }
+     }
+   }
+   ```