[feat] add configurable connection starting bitrates

docs/USER_GUIDE_EN.md

@@ -187,6 +187,22 @@ The direct IP switch in the Network group is independent of the relay settings u
   <img src="images/desktop-settings-network-en.png" width="720" alt="Network settings with Enable direct IP and an explanation of what happens when disabled" />
 </p>
 
+### Starting bandwidth configuration
+
+Quit the client, edit the `[Settings]` section in its cache directory's `config.ini`, then restart:
+
+```ini
+[Settings]
+direct_start_bitrate_bps = 1500000
+relay_start_bitrate_bps = 500000
+```
+
+Both values are in bps and set the local native media sender's initial congestion-control estimate for P2P / direct IP and TURN relay paths, respectively. Desktop defaults are 1.5 Mbps and 0.5 Mbps; the accepted range is 300000–20000000. Missing or invalid values use the corresponding default, which is written to the file on startup. Network feedback continues to adjust bandwidth, and DeskHub bandwidth budgets still apply. To change the stream you watch remotely, edit the host that sends the video. Browser connections use separate bandwidth control and are unaffected.
+
+The file is in `~/.cache/CrossDesk/` on macOS / Linux (`CrossDesk/` under `XDG_CACHE_HOME` when set), and `%LOCALAPPDATA%\CrossDesk\cache\` for installed Windows Release builds. Windows Debug builds use `cache/` under the working directory; portable builds use `data/` beside the executable.
+
+With a Windows background host configured, the INI string in `host_config` inside `%ProgramData%\CrossDeskUnattended\shared-identity.json` is authoritative. An administrator must stop both the GUI and background service before editing these two entries there, then restart both. Editing only the user cache or `host/config.ini` is overwritten by the shared configuration on startup.
+
 ### DeskHub video transmission
 
 Use **Settings → DeskHub** to configure wall sessions together. Saving applies changes immediately to current DeskHub sessions and future connections; ordinary connections are unaffected.