Add offline dependency download scripts for air-gapped environment

- download-all-dependencies.sh: Downloads Qt6 6.10.2, source tarballs, and Ubuntu .deb packages
- download-deb-packages.sh: Downloads Ubuntu .deb packages in Docker container
- verify-offline-cache.sh: Verifies offline cache completeness
- offline-cache/README.md: Documentation for offline cache structure and usage
- .gitignore: Exclude external-dependencies/offline-cache/ from version control (2.3GB)

The offline cache must be transferred to air-gapped environment via secure media.
Excluded from git due to size and security best practices.

Co-authored-by: Copilot <2235566219+Copilot@users.noreply.github.com>
Entire-Checkpoint: 01M6PFZM0GHMBF46BKYPKZC192

external-dependencies/scripts/verify-offline-cache.sh

@@ -0,0 +1,78 @@
+#!/bin/bash
+# Verify Offline Cache Integrity
+# Usage: bash verify-offline-cache.sh [offline-cache-path]
+
+set -euo pipefail
+
+OFFLINE_CACHE="${1:-./offline-cache}"
+
+echo "=== Verifying Offline Cache ==="
+echo "Path: $OFFLINE_CACHE"
+echo ""
+
+ERRORS=0
+
+# Check Qt installation
+echo "Checking Qt 6.10.2..."
+if [ -d "$OFFLINE_CACHE/qt6/6.10.2/gcc_64" ]; then
+    QT_FILES=$(find "$OFFLINE_CACHE/qt6/6.10.2/gcc_64" -type f | wc -l)
+    if [ "$QT_FILES" -ge 12000 ]; then
+        echo "✓ Qt installation complete ($QT_FILES files)"
+    else
+        echo "✗ Qt installation incomplete (expected 12000+, found $QT_FILES)"
+        ERRORS=$((ERRORS + 1))
+    fi
+else
+    echo "✗ Qt installation directory missing"
+    ERRORS=$((ERRORS + 1))
+fi
+
+# Check source tarballs
+echo ""
+echo "Checking source tarballs..."
+for tarball in KDReports-2.3.0.tar.gz Log4Qt-2.0.0.tar.gz grantlee-20d415b.tar.gz; do
+    if [ -f "$OFFLINE_CACHE/src/$tarball" ]; then
+        SIZE=$(du -h "$OFFLINE_CACHE/src/$tarball" | cut -f1)
+        echo "✓ $tarball ($SIZE)"
+    else
+        echo "✗ $tarball missing"
+        ERRORS=$((ERRORS + 1))
+    fi
+done
+
+# Check .deb packages
+echo ""
+echo "Checking Ubuntu .deb packages..."
+if [ -d "$OFFLINE_CACHE/deb" ]; then
+    DEB_COUNT=$(ls -1 "$OFFLINE_CACHE/deb"/*.deb 2>/dev/null | wc -l)
+    if [ "$DEB_COUNT" -ge 49 ]; then
+        echo "✓ Debian packages complete ($DEB_COUNT packages)"
+    else
+        echo "✗ Debian packages incomplete (expected 49+, found $DEB_COUNT)"
+        ERRORS=$((ERRORS + 1))
+    fi
+else
+    echo "✗ Debian packages directory missing"
+    ERRORS=$((ERRORS + 1))
+fi
+
+# Check package list
+echo ""
+echo "Checking package inventory..."
+if [ -f "$OFFLINE_CACHE/scripts/package-list.txt" ]; then
+    echo "✓ Package list present"
+else
+    echo "✗ Package list missing"
+    ERRORS=$((ERRORS + 1))
+fi
+
+# Summary
+echo ""
+echo "=== Verification Summary ==="
+if [ "$ERRORS" -eq 0 ]; then
+    echo "✓ All checks passed"
+    exit 0
+else
+    echo "✗ $ERRORS check(s) failed"
+    exit 1
+fi