feat: Add tablet landscape orientation support (#490)

- Tablets now support landscape mode, phones remain portrait-only
- Add OrientationAwareActivity base class for orientation management
- Add DeviceUtils.isTablet() for runtime device detection
- Update MainActivity and GeohashPickerActivity to extend OrientationAwareActivity
- Uses multiple detection criteria (screen size, density, configuration)
Fixes #480

app/src/main/java/com/bitchat/android/utils/DeviceUtils.kt

@@ -0,0 +1,39 @@
+package com.bitchat.android.utils
+
+import android.content.Context
+import android.content.res.Configuration
+import android.util.DisplayMetrics
+import android.view.WindowManager
+import androidx.core.content.getSystemService
+import kotlin.math.sqrt
+
+object DeviceUtils {
+
+    /**
+     * Determines if the current device is a tablet based on screen size and density.
+     * Uses multiple criteria to accurately detect tablets vs phones.
+     */
+    fun isTablet(context: Context): Boolean {
+        val windowManager = context.getSystemService<WindowManager>()
+        val displayMetrics = DisplayMetrics()
+        windowManager?.defaultDisplay?.getMetrics(displayMetrics)
+
+        // Calculate screen size in inches
+        val widthInches = displayMetrics.widthPixels / displayMetrics.xdpi
+        val heightInches = displayMetrics.heightPixels / displayMetrics.ydpi
+        val diagonalInches = sqrt((widthInches * widthInches) + (heightInches * heightInches))
+
+        // Check if device has tablet configuration
+        val configuration = context.resources.configuration
+        val isLargeScreen = (configuration.screenLayout and Configuration.SCREENLAYOUT_SIZE_MASK) >= Configuration.SCREENLAYOUT_SIZE_LARGE
+        val isXLargeScreen = (configuration.screenLayout and Configuration.SCREENLAYOUT_SIZE_MASK) == Configuration.SCREENLAYOUT_SIZE_XLARGE
+
+        // A device is considered a tablet if:
+        // 1. Screen diagonal is 7 inches or larger, OR
+        // 2. Configuration indicates large or xlarge screen, OR
+        // 3. Smallest width is 600dp or more (sw600dp)
+        val smallestWidthDp = context.resources.configuration.smallestScreenWidthDp
+
+        return diagonalInches >= 7.0 || isLargeScreen || isXLargeScreen || smallestWidthDp >= 600
+    }
+}