fix(playback): send Origin as a real VLC header and cover header IPC in E2E

Review follow-ups: VLC only used the Origin value as an :http-referrer
fallback while MPV already sent it via --http-header-fields; both VLC
paths (fresh spawn and RC enqueue) now emit the same
buildHttpHeaderFields list, so a real `Origin: ...` header reaches the
provider, deduplicated against an explicit headers-map Origin. The
legacy origin-as-Referer fallback stays.

The dash-clearkey Electron E2E now asserts the new IPC contract (blank
channel-level headers arrive as undefined, not empty strings) and gains
a scenario that sets a playlist-level User-Agent through the source
editor and verifies the captured MPV fallback launch carries it across
the renderer/main IPC boundary.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011id2tdJtkJYRYX8dwYYKwL

docs/architecture/m3u-playlist-module.md

@@ -969,7 +969,12 @@ wins, the playlist-level value (import dialog / playlist settings) is the
 fallback, and blank values count as absent. Every M3U external launch path
 goes through it: the auto-launch and catch-up effects in `m3u-state`, the
 manual MPV/VLC fallback in `VideoPlayerComponent`, and the embedded MPV
-payload (`embeddedPlayback`).
+payload (`embeddedPlayback`). In the main process both players emit the same
+header field list (`buildHttpHeaderFields`): a real `Origin: ...` header
+(deduplicated against the custom headers map) plus every non-empty custom
+header — MPV via `--http-header-fields`, VLC via per-input `:http-header=`
+options in both the fresh-spawn and RC-enqueue paths. VLC additionally keeps
+its legacy origin-as-Referer fallback when no Referer is set.
 
 ### DASH + ClearKey Playback