Server-side savegames: Rust service with a shared savegame list in the menu, fallback to files

- server/: small axum service storing savegames as files (1 MB each, 500 MB total, rate limit)
- docker compose: second container with a volume; nginx forwards /api/
- JS plugin: asynchronous HTTP requests polled by the game
- Save/Load open a list page (name field, new, overwrite, load, delete with confirmation)
- Without a reachable service the game keeps using savegame files and the browser autosave

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lang/en.lang

@@ -344,3 +344,25 @@ hint.punch_where = The hand can only punch heroes on your own territory.
 
 unit.chainbreaker = Chainbreaker
 hint.chainbreaker_restless = The Chainbreaker is restless: it wants something to fight!
+
+# savegames on the server
+saves.name = Name:
+saves.new = Save as new
+saves.default_name = Wave {0} - {1}
+saves.overwrite = Overwrite
+saves.load = Load
+saves.delete = Delete
+saves.sure = Sure?
+saves.meta = {3} UTC, wave {0}, map {1}, played {2}
+saves.empty = No savegames on the server yet.
+saves.loading = Loading the list ...
+saves.wait = Please wait ...
+saves.file_save = Download as file
+saves.file_load = Load from file
+cloud.deleted = Savegame deleted.
+cloud.err_net = The savegame server cannot be reached.
+cloud.err_large = The savegame is too large for the server.
+cloud.err_rate = Too many requests - please wait a moment.
+cloud.err_full = The savegame server is full.
+cloud.err_missing = This savegame no longer exists.
+cloud.err = The savegame server reported an error.