Custom Request Headers for Reverse Proxy Auth

[404Simon]

Problem

Users behind reverse proxies that want to use header-based authentication cannot log in or stream media because the client doesnt allow adding these auth headers.
I use Pangolin and really like this feature as it gives me an extra layer of security.

Solution

Adds an expandable section to the server-address entry overlay where users can configure arbitrary HTTP header key/value pairs.

Three injection points

Injection point	Scope
CefResourceRequestHandler::on_before_resource_load	All CEF browser requests (API calls, jellyfin-web)
CefURLRequest in ServerProbeClient	The server connectivity probe (HEAD + GET to /System/Info/Public)
mpv http-header-fields option	Streaming requests made directly by mpv (HLS, direct play)

What changed

• src/config/src/lib.rs: new custom_headers: Vec<(String, String)> persisted in settings.json
• src/jfn_cef/src/client_impl/resource_request.rs: CefResourceRequestHandler that injects headers into browser requests matching the configured server URL
• src/jfn_cef/src/client_impl/request.rs: CefRequestHandler that returns the resource handler per frame
• src/jfn_cef/src/client_impl.rs: wired request_handler into wrap_client!
• src/jfn_cef/src/injection.rs: added getCustomHeaders / setCustomHeaders native IPC functions
• src/jfn_cef/src/business_overlay.rs: IPC handlers for header save/load; headers on connectivity probe requests
• src/jfn_cef/src/business_web.rs: passes headers to mpv at playerLoad time
• src/mpv/src/api.rs: sync mpv_set_property_string, sets http-header-fields on the mpv handle before loadfile
• src/web/overlay.html: <details> element with header key/value rows
• src/web/overlay.css: dark-theme styling matching existing form elements
• src/web/overlay.js: dynamic header row builder, load/save via IPC
• src/web/overlay.lang.js: English i18n strings for the advanced UI

Usage

1. Open Jellyfin Desktop
2. Enter server URL
3. Click Advanced dropdown
4. Click Add Header and enter key/value pairs
5. Click Connect

Headers persist in settings.json across restarts.