feat: Wave 3 — plugin system + custom wake-word trainer

#29 Plugin system:
- New jarvis-core::plugins module: discovers user packs in
  %APPDATA%\com.priler.jarvis\plugins\<name>\command.toml so authors can
  ship voice commands without rebuilding. Sandbox capped at "standard" —
  plugins cannot escalate to "full" (which would expose `os`). Disabled
  via a `disabled` flag file. Malformed packs warn and skip; never poison
  the rest of the list. 8 unit tests.
- commands::parse_commands() merges plugins into the loaded list.
- New /plugins GUI page with enable/disable switches, error reporting,
  "Open folder" button. Tauri commands plugins_list / plugins_set_enabled
  / plugins_open_folder. Header gets a "Плагины" button. i18n keys added
  for ru/en/ua.

#8 Custom wake-word trainer wizard:
- New jarvis-core::wake_trainer module: opens its own pv_recorder
  instance, records N short PCM clips, WAV-encodes them in memory, then
  calls rustpotter's WakewordRef::new_from_sample_buffers to train and
  persist a .rpw model under %APPDATA%/com.priler.jarvis/wake_words/.
  Keepsake WAVs are also dumped for retraining later. Sanitises names to
  block path traversal. 5 unit tests.
- Settings gain `custom_wake_word: String`; listener/rustpotter.rs now
  loads the user's selected model first and always falls back to the
  bundled default so the assistant keeps working even if the custom
  file is missing.
- New /wake-trainer GUI page: stepper UI for record-sample → train,
  shows recorded count, threshold slider, refuses to start while
  jarvis-app is running (mic exclusivity). Lists existing trained
  models with size + delete button.
- 8 Tauri commands wired through (status/defaults/start/record_sample/
  finish/cancel/list_models/delete_model).

Tests: 115 → 128 (+8 plugins +5 wake_trainer). Release builds green for
all three binaries (jarvis-app / jarvis-cli / jarvis-gui).

frontend/src/components/Header.svelte

@@ -183,6 +183,10 @@
             <span class="btn-text">{t('header-memory') || 'Память'}</span>
         </button>
 
+        <button class="header-btn" on:click={() => $goto('/plugins')} title="User plugins">
+            <span class="btn-text">{t('header-plugins') || 'Плагины'}</span>
+        </button>
+
         <button class="header-btn" on:click={() => $goto('/settings')}>
             <span class="btn-text">{t('header-settings')}</span>
         </button>