Check i18n consistency (vibe-kanban) (#960)

* ## Done! ✅

The [check-i18n.sh](file:///private/var/folders/m1/9q_ct1913z10v6wbnv54j25r0000gn/T/vibe-kanban/worktrees/a6aa-check-i18n-consi/scripts/check-i18n.sh) script has been extended with key consistency checking:

**What was added:**
- `get_json_keys()` - Extracts all nested keys from translation JSON files
- `check_key_consistency()` - Validates that all languages have the same keys as English
- Both checks now run in sequence with aggregated exit status

**Features:**
- Auto-discovers languages and namespaces from the filesystem
- Reports missing keys as errors (script fails)
- Reports extra keys as warnings (optional strict mode with `I18N_FAIL_ON_EXTRA=1`)
- Verbose mode available with `I18N_VERBOSE=1`

**Current issues detected:**
- Spanish translations missing 13 keys across common.json and settings.json
- Japanese translations appear to be up to date

* Perfect! All missing Spanish translations have been added and the script now passes ✅

**Added translations:**
- [es/common.json](file:///private/var/folders/m1/9q_ct1913z10v6wbnv54j25r0000gn/T/vibe-kanban/worktrees/a6aa-check-i18n-consi/frontend/src/i18n/locales/es/common.json): 8 conversation-related keys
- [es/settings.json](file:///private/var/folders/m1/9q_ct1913z10v6wbnv54j25r0000gn/T/vibe-kanban/worktrees/a6aa-check-i18n-consi/frontend/src/i18n/locales/es/settings.json): 5 settings-related keys

frontend/src/i18n/locales/es/common.json

@@ -23,6 +23,18 @@
     "browserDefault": "Predeterminado del navegador"
   },
   "conversation": {
+    "plan": "Plan",
+    "planToggle": {
+      "show": "Mostrar plan",
+      "hide": "Ocultar plan"
+    },
+    "toolDetailsToggle": {
+      "show": "Mostrar detalles",
+      "hide": "Ocultar detalles"
+    },
+    "args": "Argumentos",
+    "output": "Salida",
+    "result": "Resultado",
     "deniedByUser": "{{toolName}} denegado por el usuario"
   },
   "branchSelector": {