Add channel key loading option to first-boot wizard

Step 3 now offers three choices:
- Skip (public mode)
- Generate new key
- Enter existing key (for joining team deployments)

Validates entered keys using Python channel module before accepting.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

rpi/first-boot-wizard.sh

@@ -179,7 +179,13 @@ This is useful if you want to share encoded images only with
 specific people (family, team, etc)."
 echo ""
 
-if gum confirm "Generate a private channel key?" --default=false; then
+CHANNEL_CHOICE=$(gum choose \
+  "Skip (public mode)" \
+  "Generate new key" \
+  "Enter existing key")
+
+case "$CHANNEL_CHOICE" in
+  "Generate new key")
     echo ""
     # Generate key to temp file (gum spin doesn't capture stdout well)
     KEY_FILE=$(mktemp)
@@ -221,10 +227,52 @@ if gum confirm "Generate a private channel key?" --default=false; then
       echo ""
       gum confirm "Continue" --default=true --affirmative="OK" --negative=""
     fi
-else
+    ;;
+
+  "Enter existing key")
+    echo ""
+    gum style --foreground 245 "Enter the channel key from your team/deployment."
+    gum style --foreground 245 "Format: XXXX-XXXX-XXXX-XXXX-XXXX-XXXX-XXXX-XXXX"
+    echo ""
+
+    while true; do
+      ENTERED_KEY=$(gum input --placeholder "ABCD-1234-EFGH-5678-IJKL-9012-MNOP-3456" --width 50)
+
+      if [ -z "$ENTERED_KEY" ]; then
+        gum style --foreground 214 "→ Cancelled, using public mode"
+        CHANNEL_KEY=""
+        break
+      fi
+
+      # Validate the key using Python
+      VENV_PYTHON="$INSTALL_DIR/venv/bin/python"
+      if "$VENV_PYTHON" -c "from stegasoo.channel import validate_channel_key, format_channel_key; k='$ENTERED_KEY'; exit(0 if validate_channel_key(k) else 1)" 2>/dev/null; then
+        # Get formatted key
+        CHANNEL_KEY=$("$VENV_PYTHON" -c "from stegasoo.channel import format_channel_key; print(format_channel_key('$ENTERED_KEY'))" 2>/dev/null)
+        echo ""
+        gum style --foreground 82 "✓ Channel key accepted!"
+        gum style --foreground 245 "Key: $CHANNEL_KEY"
+        break
+      else
+        echo ""
+        gum style --foreground 196 "Invalid key format. Please check and try again."
+        gum style --foreground 245 "Expected: 32 alphanumeric characters (with or without dashes)"
+        echo ""
+        if ! gum confirm "Try again?" --default=true; then
           gum style --foreground 214 "→ Using public mode"
+          CHANNEL_KEY=""
+          break
+        fi
+      fi
+    done
+    ;;
+
+  *)
+    gum style --foreground 214 "→ Using public mode"
+    CHANNEL_KEY=""
     sleep 0.5
-fi
+    ;;
+esac
 
 # =============================================================================
 # Step 4: Overclock Configuration