fix: TTS hot-swap actually works + GUI max-width + Python builder discoverable

ROOT CAUSE ANALYSIS

User reported: (a) TTS dropdown doesn't apply, (b) GUI feels stretched,
(c) no link to command builder, (d) Python doesn't even start.

Issues (a), (b), (c) addressed here. (d) addressed in python repo.

=== Fix A: TTS hot-swap (was completely broken) ===

Two distinct bugs:
1. `tts::init_backend()` only read JARVIS_TTS env var, NEVER consulted
   `Settings.tts_backend` in the DB. So saving "Silero" in the GUI
   dropdown changed nothing — not even on restart.
2. `BACKEND: OnceCell<...>` couldn't be replaced at runtime, so even
   if init had picked the right backend, the dropdown was useless
   mid-session.

Fixed:
- `tts::BACKEND` is now `Lazy<RwLock<Option<Arc<dyn TtsBackend>>>>`.
- `choose_backend_name()` resolves from DB → env → auto-detect.
- New `swap_to(name)` replaces the live backend atomically.
- IPC: new `IpcAction::SwitchTts { backend }`. jarvis-app reacts to
  it the same way it reacts to SwitchLlm.
- GUI's `set_tts_backend` now persists + hot-swaps GUI's TTS + Svelte
  page also fires `switchDaemonTts()` over WS so the running daemon
  installs the new backend.
- Removed "Применится при следующем запуске" hint — now applies
  immediately. RU + EN locale strings updated.

=== Fix B: GUI max-width ===

main + #header had no max-width, so on a 1920px monitor the labels
sprawled across 1860px of space, which felt stretched. Added
max-width: 1280px + margin: 0 auto + box-sizing: border-box. Smaller
windows are unaffected.

Also added the same cap on .app-container.assist-page so the home
screen arc reactor centres properly.

=== Fix C: Command builder discoverable ===

The Python fork ships a separate yaml-editing tool at
`python/tools/command_builder/` (pywebview GUI). No way to find it
from the main Rust GUI. Fixed:

- New tauri command `open_command_builder` (crates/jarvis-gui/src/
  tauri_commands/builder.rs). Locates the Python fork via
  $JARVIS_PYTHON_DIR → sibling python/ → C:\Jarvis\python, prefers
  the project's .venv, spawns `python -m tools.command_builder`.
- New blue "Конструктор команд (Python)" button on /settings page
  near the Save / Back buttons.

Tests: 140 rust + 104 python all pass. Release builds of jarvis-app
and jarvis-gui both green.

crates/jarvis-core/src/i18n/locales/ru.ftl

@@ -153,7 +153,7 @@ settings-ai-tips = Подсказки
 settings-llm-backend = LLM движок
 settings-llm-backend-desc = Где исполнять LLM-запросы. Hot-swap, без рестарта.
 settings-tts-backend = TTS движок
-settings-tts-backend-desc = Применится при следующем запуске jarvis-app.
+settings-tts-backend-desc = Применяется сразу — без перезапуска.
 settings-llm-context = Контекст разговора
 settings-llm-context-desc = LLM помнит предыдущие реплики. Сбросить, если ответы становятся странными.
 settings-llm-reset = Сбросить контекст