Persistent HEADLESS-1 + SSH-tunnel-friendly cert SANs + web UI lockdown

Two streams of fixes shipped together.

Headless persistence (root cause of "Fatal: Unable to find display or
encoder during startup")
- bin/sunshine-stream-undo.sh: stop removing HEADLESS-1 on disconnect.
  Create-on-connect / destroy-on-disconnect raced with Sunshine's startup
  encoder probe and made every restart fail with a fatal-but-misleading
  warning. The output now lives across stream sessions; sunshine-stream-
  do.sh just resizes it per client.
- files/headless-prestart.conf: systemd-user drop-in that runs
  'hyprctl output create headless' (non-fatal) before Sunshine starts, so
  HEADLESS-1 exists before the encoder probe.
- lib/headless.sh: install_headless_prestart_dropin resolves the actual
  unit name (sunshine.service or app-dev.lizardbyte.app.Sunshine.service)
  and lands the drop-in under ~/.config/systemd/user/<unit>.d/.
- lib/service.sh: enable_sunshine_service calls install_headless_prestart_
  dropin when STREAM_MODE=headless. Placed after ensure_sunshine_unit_
  present so the unit name is settled when the drop-in is written.
- install.sh: comment noting the drop-in install is deferred to the
  service-enable step.

Web UI lockdown + tunnel-friendly certs
- lib/config.sh: emits origin_web_ui_allowed = pc. Sunshine rejects web UI
  requests from anywhere other than localhost regardless of bind address.
  Streaming/pairing (47989) stays LAN-accessible. Inline comment documents
  the SSH tunnel recipe.
- lib/certs.sh: add DNS:localhost and IP:127.0.0.1 to host cert SANs so
  the tunneled https://localhost:47990 URL doesn't trigger a hostname
  mismatch. Idempotency check now requires those SANs too.

Misc.
- files/sunshine.service: fallback unit also gains the prestart ExecStartPre.
- lib/service.sh: ensure_sunshine_unit_present aliases the reverse-DNS
  Sunshine unit as sunshine.service when sunshine-bin's short-name unit
  isn't installed.

files/sunshine.service

@@ -0,0 +1,18 @@
+[Unit]
+Description=Self-hosted game stream host for Moonlight
+StartLimitIntervalSec=500
+StartLimitBurst=5
+PartOf=graphical-session.target
+After=graphical-session.target
+
+[Service]
+Type=simple
+# Ensure the Hyprland headless output exists before Sunshine probes encoders.
+# Non-fatal if Hyprland isn't reachable (e.g., service starts before login).
+ExecStartPre=-/usr/bin/hyprctl output create headless
+ExecStart=/usr/bin/sunshine
+Restart=on-failure
+RestartSec=5s
+
+[Install]
+WantedBy=graphical-session.target