feat: add cross-platform support via platform detection module

Introduces src/platform.rs with OS detection and env var overrides.
All hardcoded Linux paths replaced with Platform::detect() across
8 source files. Key changes:

- New Platform struct with 11 fields (all overridable via env vars)
- /proc/ access gated to Linux (#[cfg(target_os = "linux")])
- pgrep/pkill patterns broadened for cross-platform LS discovery
- sec-ch-ua-platform header now dynamic per OS
- Token, traces, config, CA cert paths use platform module
- LD_PRELOAD DNS redirect gated to Linux only
- Setup scripts for Linux (systemd) and macOS (launchd)
- find_ls_binary_path has cross-platform stubs

All 46 tests pass, cargo check clean.

src/constants.rs

@@ -21,26 +21,54 @@ struct DetectedVersions {
 /// back to its binary, then walking up to the app root. Falls back to
 /// well-known install paths.
 fn find_install_dir() -> Option<String> {
-    // 1. Try tracing the running language server → /usr/share/antigravity/resources/app/extensions/...
-    if let Ok(output) = Command::new("sh")
-        .args(["-c", "pgrep -f language_server_linux | head -1"])
-        .output()
+    let p = crate::platform::Platform::detect();
+
+    // 1. Check if platform-detected app_root exists
+    let app_root_parent = std::path::Path::new(&p.app_root)
+        .parent()
+        .and_then(|p| p.parent())
+        .map(|p| p.to_string_lossy().to_string());
+    if let Some(ref dir) = app_root_parent {
+        if fs::metadata(format!("{dir}/resources/app/product.json")).is_ok() {
+            return Some(dir.clone());
+        }
+    }
+
+    // 2. Try tracing the running language server via /proc (Linux only)
+    #[cfg(target_os = "linux")]
     {
-        let pid = String::from_utf8_lossy(&output.stdout).trim().to_string();
-        if !pid.is_empty() {
-            if let Ok(exe) = fs::read_link(format!("/proc/{pid}/exe")) {
-                let exe_str = exe.to_string_lossy().to_string();
-                // exe is like: /usr/share/antigravity/resources/app/extensions/antigravity/bin/language_server_linux_x64
-                // We want: /usr/share/antigravity
-                if let Some(idx) = exe_str.find("/resources/") {
-                    return Some(exe_str[..idx].to_string());
+        if let Ok(output) = Command::new("sh")
+            .args(["-c", "pgrep -f language_server | head -1"])
+            .output()
+        {
+            let pid = String::from_utf8_lossy(&output.stdout).trim().to_string();
+            if !pid.is_empty() {
+                if let Ok(exe) = fs::read_link(format!("/proc/{pid}/exe")) {
+                    let exe_str = exe.to_string_lossy().to_string();
+                    if let Some(idx) = exe_str.find("/resources/") {
+                        return Some(exe_str[..idx].to_string());
+                    }
                 }
             }
         }
     }
 
-    // 2. Fall back to well-known install paths
-    for path in &["/usr/share/antigravity", "/opt/Antigravity"] {
+    // 3. Fall back to well-known install paths
+    #[cfg(target_os = "linux")]
+    let candidates = ["/usr/share/antigravity", "/opt/Antigravity"];
+    #[cfg(target_os = "macos")]
+    let candidates = [
+        "/Applications/Antigravity.app/Contents",
+        &format!("{}/Applications/Antigravity.app/Contents", std::env::var("HOME").unwrap_or_default()),
+    ];
+    #[cfg(target_os = "windows")]
+    let candidates = [
+        &format!("{}\\Programs\\Antigravity", std::env::var("LOCALAPPDATA").unwrap_or_default()),
+    ];
+    #[cfg(not(any(target_os = "linux", target_os = "macos", target_os = "windows")))]
+    let candidates: [&str; 0] = [];
+
+    for path in &candidates {
         if fs::metadata(format!("{path}/resources/app/product.json")).is_ok() {
             return Some(path.to_string());
         }
@@ -178,14 +206,23 @@ pub const LS_SERVICE: &str = "exa.language_server_pb.LanguageServerService";
 
 /// Log base directory for Antigravity.
 pub fn log_base() -> String {
-    let home = std::env::var("HOME").unwrap_or_else(|_| "/root".to_string());
-    format!("{home}/.config/Antigravity/logs")
+    let p = crate::platform::Platform::detect();
+    // Antigravity logs live next to its state DB
+    let state_parent = std::path::Path::new(&p.state_db_path)
+        .parent()
+        .and_then(|p| p.parent())
+        .and_then(|p| p.parent())
+        .map(|p| p.to_string_lossy().to_string())
+        .unwrap_or_else(|| {
+            let home = std::env::var("HOME").unwrap_or_else(|_| "/root".to_string());
+            format!("{home}/.config/Antigravity")
+        });
+    format!("{state_parent}/logs")
 }
 
 /// Token file path.
 pub fn token_file_path() -> String {
-    let home = std::env::var("HOME").unwrap_or_else(|_| "/root".to_string());
-    format!("{home}/.config/zerogravity/token")
+    crate::platform::Platform::detect().token_path.to_string_lossy().to_string()
 }
 
 /// User-Agent string matching the Electron webview — computed once.