feat: persistent DB host config, async DB ops, Exit button, test connection

- Save/load DB host from config.json (works both in dev and built .exe)
- All DB operations moved to background threads to prevent UI blocking
- Connection timeout set to 5s to avoid long hangs on unreachable host
- Added Test Connection button to Settings popup
- Added red Exit button fixed to bottom-right corner of main screen
- Updated DatabaseApp.spec with full Kivy deps (sdl2, glew, angle) and hidden imports
- PyInstaller-aware base path using sys.frozen for config.json persistence

DatabaseApp.spec

@@ -1,12 +1,36 @@
 # -*- mode: python ; coding: utf-8 -*-
-
+from kivy_deps import sdl2, glew, angle
 
 a = Analysis(
     ['main.py'],
     pathex=[],
     binaries=[],
     datas=[],
-    hiddenimports=['mysql.connector', 'kivy.core.window.window_sdl2', 'win32timezone'],
+    hiddenimports=[
+        'mysql.connector',
+        'mysql.connector.locales',
+        'mysql.connector.locales.eng',
+        'kivy',
+        'kivy.core.window',
+        'kivy.core.window.window_sdl2',
+        'kivy.core.text',
+        'kivy.core.text.text_sdl2',
+        'kivy.core.image',
+        'kivy.core.image.img_sdl2',
+        'kivy.core.image.img_pil',
+        'kivy.core.audio',
+        'kivy.core.clipboard',
+        'kivy.core.clipboard.clipboard_sdl2',
+        'kivy.core.spelling',
+        'kivy.graphics',
+        'kivy.graphics.cgl_backend',
+        'kivy.graphics.cgl_backend.cgl_glew',
+        'kivy.input.providers',
+        'kivy.input.providers.wm_touch',
+        'kivy.input.providers.wm_pen',
+        'win32timezone',
+        'pkg_resources.py2_compat',
+    ],
     hookspath=[],
     hooksconfig={},
     runtime_hooks=[],
@@ -21,6 +45,7 @@ exe = EXE(
     a.scripts,
     a.binaries,
     a.datas,
+    *[Tree(p) for p in (sdl2.dep_bins + glew.dep_bins + angle.dep_bins)],
     [],
     name='DatabaseApp',
     debug=False,