feat: Wave 3 — plugin loader + Flet GUI (parity with Rust fork)

Plugin loader (mirrors Rust #29):
- plugins_store.py walks %APPDATA%\com.priler.jarvis\plugins\<name>\
  for commands.yaml fragments and merges them into VA_CMD_LIST at
  startup. Same schema as the project root commands.yaml. Per-pack
  `disabled` flag file skips a pack without deletion. Built-in ids
  win conflicts so a malicious pack can't silently override
  open_browser. 9 unit tests (parity with the Rust tests).
- main.py: plugin merge runs after the root yaml load; logs the
  number of extra commands picked up.

Flet GUI (#27 — first parity with the Rust Tauri GUI):
- New gui/ package: __main__.py launches via `python -m gui` (or
  gui.bat). app.py routes between 7 pages (Главная / Команды /
  Макросы / Расписание / Память / Плагины / Настройки) via
  NavigationRail; dark theme matching the Tauri look.
- gui/services.py centralises every store-module call the pages
  share — single place to swap implementations later.
- One file per page in gui/pages/, all expose `build(page)` and use
  the same card layout idiom. Pages mirror the Tauri ones feature-
  for-feature: home (status + counters + launcher), commands
  (filterable list of all VA_CMD_LIST entries), macros (list +
  delete + recording banner), scheduler (list + remove), memory
  (add/forget/search), plugins (toggle/open folder), settings
  (profile + LLM hot-swap).
- requirements.txt: flet>=0.85 (optional; assistant runs without).
- 3 smoke tests verify imports + build callables; skip cleanly when
  Flet isn't installed.

README: documents both the GUI and the plugin layout in user-facing
terms. Tests: 42 → 54 (+9 plugins +3 GUI smoke).

main.py

@@ -19,6 +19,7 @@ import webrtcvad
 import yaml
 
 import extensions  # new-command handlers ported from rust fork
+import plugins_store  # user plugin packs under APP_CONFIG_DIR/plugins/
 from comtypes import CLSCTX_ALL
 from fuzzywuzzy import fuzz
 from pvrecorder import PvRecorder
@@ -38,6 +39,18 @@ VA_CMD_LIST = yaml.safe_load(
     open('commands.yaml', 'rt', encoding='utf8'),
 )
 
+# Merge in any user-installed plugin packs from APP_CONFIG_DIR/plugins/.
+# Built-in ids take precedence (plugins cannot override stock commands).
+_plugin_cmds = plugins_store.discover()
+_added = 0
+for _cmd_id, _entry in _plugin_cmds.items():
+    if _cmd_id in VA_CMD_LIST:
+        continue
+    VA_CMD_LIST[_cmd_id] = _entry
+    _added += 1
+if _added > 0:
+    print(f"[plugins] loaded {_added} extra command(s) from APP_CONFIG_DIR/plugins/")
+
 system_message = {"role": "system", "content": (
     "Ты — J.A.R.V.I.S. (Just A Rather Very Intelligent System), ИИ-ассистент Тони Старка из киновселенной Marvel "
     "(до событий Age of Ultron — ты НЕ Vision и НЕ FRIDAY). "