Add an in-app User Guide and standalone PDF

Adds docs/USER_GUIDE.md, a 10-section end-user guide (getting
started, managing/organizing profiles, SSH and RDP connections,
session management, settings, troubleshooting). It's embedded into
the app binary via a Qt resource file and rendered by a new
Help -> User Guide window: a topic sidebar plus content pane, not a
single scrolling document, with cross-reference links between
sections routed to sidebar selection rather than relying on Qt's
Markdown importer's lack of heading anchors.

A separate, non-shipped tool (tools/user-guide-pdf/) renders the
same source to a standalone PDF via QTextDocument + QPrinter,
wrapped by packaging/docs/build-user-guide-pdf.sh. Kept fully
outside the main CMake target so Qt6::PrintSupport never becomes a
runtime dependency of the shipped app (confirmed via ldd). The PDF
itself isn't committed -- generated per release like the platform
installers.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

packaging/docs/build-user-guide-pdf.sh

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+ROOT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)"
+BUILD_DIR="${1:-$ROOT_DIR/build-doc-tool}"
+OUTPUT_PATH="${2:-$ROOT_DIR/docs/USER_GUIDE.pdf}"
+
+cmake -S "$ROOT_DIR/tools/user-guide-pdf" -B "$BUILD_DIR" -G Ninja
+cmake --build "$BUILD_DIR"
+
+QT_QPA_PLATFORM=offscreen "$BUILD_DIR/user-guide-pdf" \
+    "$ROOT_DIR/docs/USER_GUIDE.md" \
+    "$OUTPUT_PATH"
+
+echo "Created $OUTPUT_PATH"