Implement git commit hash tracking for build display

- Add get_git_commit() method to read /var/lib/cm-dashboard/git-commit
- Replace NixOS build version with actual git commit hash
- Show deployed commit hash as 'Build:' value for accurate tracking
- Enable verification of which exact commit is deployed per host
- Update version to 0.1.42

dashboard/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "cm-dashboard"
-version = "0.1.41"
+version = "0.1.42"
 edition = "2021"
 
 [dependencies]

dashboard/src/main.rs

@@ -14,7 +14,7 @@ use app::Dashboard;
 
 /// Get hardcoded version
 fn get_version() -> &'static str {
-    "v0.1.41"
+    "v0.1.42"
 }
 
 /// Check if running inside tmux session