feat(#350): pairing login flow with QR scan + manual entry (#421)

* feat(#350): pairing login flow with QR scan + manual entry

Adds PairingCodeEntryScreen with CameraX + ML Kit QR scanning
and manual code entry. The screen parses hermes://connect URIs,
Base64-encoded JSON, and raw token formats, validates via
/api/status, and saves credentials on success.

- New PairingCodeEntryScreen + PairingCodeEntryViewModel
- CameraX 1.4.1 + ML Kit barcode-scanning 17.3.0
- CAMERA permission + proguard rules
- Landing screen Pairing Login button wired correctly

* fix(#350): resolve smart-cast on delegated property for errorMessage

Co-authored-by: Hy4ri <hy4ri@users.noreply.github.com>

* refactor(#421): remove QR scanner from pairing login

- Remove CameraX + ML Kit barcode dependencies
- Drop CAMERA permission and camera feature from manifest
- Strip QrScannerTab, QrCodeScannerView, processFrame composables
- Remove camera permission state/methods from ViewModel
- Purge all QR/camera strings from strings.xml
- Delete ML Kit proguard rules

The pairing login now shows only the manual code entry UI.

---------

Co-authored-by: M57 <hy4ri@users.noreply.github.com>

app/src/main/java/com/m57/hermescontrol/NavigationKeys.kt

@@ -27,6 +27,8 @@ import kotlinx.serialization.Serializable
 
 @Serializable data object PairingScreen : NavKey
 
+@Serializable data object PairingCodeEntryScreen : NavKey
+
 @Serializable data object ConfigScreen : NavKey
 
 @Serializable data object McpServersScreen : NavKey