Harden Linux server runtime and packaging

Adds end-to-end Linux compatibility work for the dedicated server: new CI workflow (Ubuntu + Arch), line-ending/executable safeguards, and a local venv-first startup flow with split dependency files for server vs optional host GUI tooling. Improves runtime resilience with better bind error messages, stricter config validation, writable-state checks, cleaner socket/thread shutdown behavior, and SIGTERM-aware graceful stop handling for headless/systemd use. Updates deployment/startup docs and adds portability/runtime integration tests to lock in these behaviors.

.gitattributes

@@ -0,0 +1,45 @@
+# Normalize text files by default; platform-specific EOL below.
+* text=auto
+
+# Shell / service / config (LF)
+*.sh text eol=lf
+*.py text eol=lf
+*.service text eol=lf
+*.conf text eol=lf
+*.json text eol=lf
+*.toml text eol=lf
+*.yaml text eol=lf
+*.yml text eol=lf
+*.md text eol=lf
+*.txt text eol=lf
+*.cmake text eol=lf
+*.qrc text eol=lf
+*.h text eol=lf
+*.cpp text eol=lf
+*.c text eol=lf
+*.hpp text eol=lf
+CMakeLists.txt text eol=lf
+
+# Windows scripts (CRLF)
+*.bat text eol=crlf
+*.cmd text eol=crlf
+*.ps1 text eol=crlf
+
+# Binary
+*.png binary
+*.jpg binary
+*.jpeg binary
+*.gif binary
+*.ico binary
+*.exe binary
+*.dll binary
+*.zip binary
+*.tar binary
+*.gz binary
+*.lib binary
+*.pdb binary
+*.obj binary
+*.o binary
+*.a binary
+*.so binary
+*.dylib binary