feat(setup): auto-detect npx path during installation

The setup wizard now auto-detects the npx/npx.cmd path via
`which npx` (Unix) or `where npx.cmd` (Windows) and writes it
to [browser].node_path in config.toml.

Changes:
- detect_npx_path(): cross-platform detection using which/where
- SerializedConfig now includes optional [browser] section
- Flat config parser (main.rs) handles [browser] section
- browser: BrowserSection threaded through AgentConfig everywhere
- init_browser() accepts node_path parameter with proper fallback

crates/gyandoot-tui/src/setup.rs

@@ -424,6 +424,8 @@ fn write_config_file(config: &ProviderConfig) -> anyhow::Result<PathBuf> {
 struct SerializedConfig {
     provider: SerializedProvider,
     agent: SerializedAgent,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    browser: Option<SerializedBrowser>,
 }
 
 #[derive(Serialize)]
@@ -445,10 +447,46 @@ struct SerializedAgent {
     name: String,
 }
 
+/// Serializable [browser] section for TOML output.
+#[derive(Serialize)]
+struct SerializedBrowser {
+    #[serde(skip_serializing_if = "Option::is_none")]
+    node_path: Option<String>,
+}
+
 fn is_false(v: &bool) -> bool {
     !v
 }
 
+/// Auto-detect the path to npx/npx.cmd.
+///
+/// Tries `which npx` on Unix, `where npx.cmd` on Windows.
+/// Returns `None` if not found on PATH.
+fn detect_npx_path() -> Option<String> {
+    let (prog, arg) = if cfg!(target_os = "windows") {
+        ("where", "npx.cmd")
+    } else {
+        ("which", "npx")
+    };
+
+    std::process::Command::new(prog)
+        .arg(arg)
+        .output()
+        .ok()
+        .and_then(|out| {
+            if out.status.success() {
+                let path = String::from_utf8_lossy(&out.stdout)
+                    .lines()
+                    .next()?
+                    .trim()
+                    .to_string();
+                if !path.is_empty() { Some(path) } else { None }
+            } else {
+                None
+            }
+        })
+}
+
 impl From<&ProviderConfig> for SerializedConfig {
     fn from(c: &ProviderConfig) -> Self {
         let kind_str = match c.kind {
@@ -475,6 +513,9 @@ impl From<&ProviderConfig> for SerializedConfig {
             agent: SerializedAgent {
                 name: "gyandoot".to_string(),
             },
+            browser: detect_npx_path().map(|np| SerializedBrowser {
+                node_path: Some(np),
+            }),
         }
     }
 }