fix: Add missing keyboard_backends package to pyproject.toml

The vocalinux.ui.keyboard_backends module was not being installed because
it wasn't listed in the packages configuration. This caused a ModuleNotFoundError
when trying to launch Vocalinux after installation.

Fixes installation issue where the app fails to start with:
ModuleNotFoundError: No module named 'vocalinux.ui.keyboard_backends'

pyproject.toml

@@ -16,7 +16,7 @@ maintainers = [
 ]
 requires-python = ">=3.8"
 keywords = [
-    "voice", "dictation", "speech", "recognition", 
+    "voice", "dictation", "speech", "recognition",
     "linux", "accessibility", "voice-typing", "speech-to-text"
 ]
 classifiers = [
@@ -85,7 +85,7 @@ Issues = "https://github.com/jatinkrmalik/vocalinux/issues"
 Changelog = "https://github.com/jatinkrmalik/vocalinux/releases"
 
 [tool.setuptools]
-packages = ["vocalinux", "vocalinux.speech_recognition", "vocalinux.text_injection", "vocalinux.ui", "vocalinux.utils"]
+packages = ["vocalinux", "vocalinux.speech_recognition", "vocalinux.text_injection", "vocalinux.ui", "vocalinux.ui.keyboard_backends", "vocalinux.utils"]
 package-dir = {"" = "src"}
 
 [tool.setuptools.dynamic]