feat(coop): config-driven multi-rendezvous server selection

[NonPolynomialTim]

The main reason for this change is that the repo immediately gives an error when pressing the "COOP" button if the rendezvous server keys are not compiled into the build, locking players out of TCP as well. This fixes that by gracefully failing, while future-proofing UDP should the current rendezvous server cease to exist for whatever reason—the community can now host their own.

Move the rendezvous host/ports/version/public-keys out of source into a runtime rendezvous.json holding multiple named servers. The Server Browser gains a top-right combobox to pick the active server; all coop operations use the selection. rendezvous.cfg goes in the root of the build directory, next to OpenXcom.exe.

• rendezvous_config: JSON servers[], atomic active index, per-index accessor; active-server accessors keep their signatures so existing glue is unchanged
• DisableableComboBox: ComboBox subclass with per-option enable/disable (setColor face + setTextColor label kept independent)
• glue: probeRendezvousServer / probeAllRendezvousServersAsync (punch-free LIST_ROOMS health check)
• ServerList: combobox top-right; parallel probes on entry; per-server status drives "(Wait...)"/"(offline)" labels, greyed disabled rows, dimmed button label when the selection is offline, and an animated "Fetching server list ..." / offline warning in a shared textbox; selection persisted by name in rendezvous_selection.json with fallback to the first server
• title left-aligned to the left-column widgets

Waiting:

All servers down:

Server up: