feat(screenshots): add headless-Chrome capture pipeline with fake speaker

Refreshes docs/images/ui-{settings,devices,sync,migration}.png by
driving the web UI in chromedp against a synthetic speaker, so
documentation can be regenerated without real hardware and without
leaking personal data from the local network.

Three independent pieces:

- pkg/service/testing/fakespeaker — embeddable library serving the
  HTTP and telnet surface the migration wizard probes (/info,
  /presets, /recents and a getpdo CurrentSystemConfiguration reply
  that places the device on the unmigrated happy path).
- cmd/dummy-speaker — thin CLI wrapping the library; self-registers
  with a running service via POST /setup/devices.
- scripts/screenshots — chromedp runner driven by a JSON manifest;
  decoupled from speaker/service setup so it can target any backend
  URL. run.sh orchestrates a one-shot end-to-end capture and seeds
  settings.json with a generic hostname plus discovery disabled to
  keep real-network state out of the captures.

Captures are at DPR=2 for retina-sharp text.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pkg/service/testing/fakespeaker/testdata/presets.xml

@@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="UTF-8" ?>
+<presets>
+    <preset id="1" createdOn="1700000000" updatedOn="1700000000">
+        <ContentItem source="TUNEIN" type="stationurl" location="/v1/playback/station/s24939" sourceAccount="" isPresetable="true">
+            <itemName>Demo Radio</itemName>
+            <containerArt>https://example.invalid/preset1.jpg</containerArt>
+        </ContentItem>
+    </preset>
+    <preset id="2" createdOn="1700000000" updatedOn="1700000000">
+        <ContentItem source="TUNEIN" type="stationurl" location="/v1/playback/station/s00000" sourceAccount="" isPresetable="true">
+            <itemName>Demo News</itemName>
+            <containerArt>https://example.invalid/preset2.jpg</containerArt>
+        </ContentItem>
+    </preset>
+</presets>