fix(install): render the systemd units from their templates, not from heredocs (#547)

* fix(install): render the systemd units from their templates, not from heredocs

The installers carried their own inline copies of units that also exist as
templates under systemd/, and the copies drifted.

install_service.sh renders ledmatrix.service from the template correctly, then
wrote ledmatrix-web.service from a heredoc that predated it -- missing
Wants=network-online.target, RestartSec=10, SyslogIdentifier, CacheDirectory,
CacheDirectoryMode and Environment=USE_THREADING=1. install_web_service.sh had
a third copy, and install_wifi_monitor.sh a fourth, that one already differing
from its template (syslog where the template says journal).

startup_validator.py compares the installed unit against the template, so a
rig installed this way warned on every boot -- and the remedy the warning
names, "re-run scripts/install/install_service.sh", reinstalled the same stale
copy. The warning could never clear. Reproduced on a live rig running exactly
that unit.

All three installers now render systemd/*.service through the same placeholder
substitution. The template gains a __USER__ placeholder rather than hardcoding
User=root, because the web interface runs as whoever installed it.

That last point was a second, independent cause of a permanent warning: the
validator substituted a fixed "root", so any non-root install reported drift
forever. It now reads User= from the installed unit -- an install-time
decision, not something the template dictates -- and compares everything else
strictly. first_time_install.sh already reads the installed User= the same way.

Tests cover a non-root web unit not warning, a genuinely changed directive in
that unit still warning, the User= fallback, and a grep-based guard that no
installer under scripts/install/ contains an inline unit body. That guard is
what found the install_wifi_monitor.sh copy.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014RRtqXDCnvnY6EQwhT5CV9

* fix(install): escape sed replacements, use mktemp, and make render failures fatal

Address CodeRabbit findings on install_service.sh, install_web_service.sh and
install_wifi_monitor.sh:

- Values interpolated into each script's sed expression (project root path,
  username) were not escaped, so a value containing &, \ or the | delimiter
  would corrupt the rendered systemd unit. Add a shared
  sed_escape_replacement() helper in the new scripts/install/lib_systemd_render.sh
  (sourced by all three scripts) and apply it to every sed replacement.
- install_service.sh rendered the main and web units to the predictable path
  /tmp/ledmatrix.service.tmp before installing them -- a symlink/TOCTOU race
  (CWE-377). Use mktemp for both, with a trap to clean up on exit.
- install_service.sh treated a missing template as a mere warning and then
  checked only whether a unit already existed at the destination before
  enabling/starting it, so a render failure could silently fall back to
  enabling a stale, previously-installed unit. Both unit blocks now exit
  non-zero on a missing template or a failed render.

Also rename the ambiguous loop variable `l` to `line` in
test/test_systemd_unit_drift.py (Ruff E741); ruff isn't wired into any CI
workflow in this repo today, so this isn't currently CI-blocking, but the
rename is trivial and correct regardless.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S3bPMESe2TfrGvbs1ef9c5

* test(install): cover sed_escape_replacement against sed-special characters

CodeRabbit asked for regression coverage using a project path containing an
ampersand; the earlier commits on this branch already fixed the escaping,
mktemp usage, and enable/start-on-fatal-render-failure findings, and the
l->line rename was already applied -- this closes the one remaining gap.

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

---------

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/install/install_wifi_monitor.sh

@@ -18,6 +18,9 @@ USER_HOME=$(eval echo ~$ACTUAL_USER)
 # Determine the Project Root Directory (parent of scripts/install/)
 PROJECT_ROOT_DIR=$(cd "$(dirname "$0")/../.." && pwd)
 
+# shellcheck source=scripts/install/lib_systemd_render.sh
+source "$PROJECT_ROOT_DIR/scripts/install/lib_systemd_render.sh"
+
 echo "Installing LED Matrix WiFi Monitor Service for user: $ACTUAL_USER"
 echo "Using home directory: $USER_HOME"
 echo "Project root directory: $PROJECT_ROOT_DIR"
@@ -64,30 +67,19 @@ if [ ${#MISSING_PACKAGES[@]} -gt 0 ]; then
     echo "✓ Package installation completed"
 fi
 
-# Create service file with correct paths
+# Render the unit from systemd/ledmatrix-wifi-monitor.service rather than
+# inlining a second copy here. The copy this replaced had already drifted --
+# it wrote StandardOutput/StandardError=syslog where the template says journal.
 echo ""
 echo "Creating systemd service file..."
-SERVICE_FILE_CONTENT=$(cat <<EOF
-[Unit]
-Description=LED Matrix WiFi Monitor Daemon
-After=network.target
-Wants=network.target
+TEMPLATE="$PROJECT_ROOT_DIR/systemd/ledmatrix-wifi-monitor.service"
+if [ ! -f "$TEMPLATE" ]; then
+    echo "ERROR: unit template not found at $TEMPLATE"
+    exit 1
+fi
 
-[Service]
-Type=simple
-User=root
-WorkingDirectory=$PROJECT_ROOT_DIR
-ExecStart=/usr/bin/python3 $PROJECT_ROOT_DIR/scripts/utils/wifi_monitor_daemon.py --interval 30
-Restart=on-failure
-RestartSec=10
-StandardOutput=syslog
-StandardError=syslog
-SyslogIdentifier=ledmatrix-wifi-monitor
-
-[Install]
-WantedBy=multi-user.target
-EOF
-)
+ESCAPED_PROJECT_ROOT_DIR=$(sed_escape_replacement "$PROJECT_ROOT_DIR")
+SERVICE_FILE_CONTENT=$(sed "s|__PROJECT_ROOT_DIR__|$ESCAPED_PROJECT_ROOT_DIR|g; s|__USER__|root|g" "$TEMPLATE")
 
 if [ "$EUID" -eq 0 ]; then
     echo "$SERVICE_FILE_CONTENT" | tee /etc/systemd/system/ledmatrix-wifi-monitor.service > /dev/null