feat(proxy): validate proxy URL before saving setting

Add scheme and host validation to the proxy set command. Only
http, https, socks5, socks5h, socks4, and socks4a schemes are
accepted. Invalid URLs are rejected with a helpful error message
listing supported formats.

This prevents users from accidentally saving malformed proxy
addresses that would silently fail at download time.

Author: lispking

src/command/proxy.zig

@@ -5,6 +5,27 @@
 
 const std = @import("std");
 const zvm_mod = @import("../core/zvm.zig");
+const terminal = @import("../core/terminal.zig");
+
+/// Supported proxy schemes.
+const valid_schemes = [_][]const u8{ "http://", "https://", "socks5://", "socks5h://", "socks4://", "socks4a://" };
+
+/// Validate that a proxy URL has a supported scheme and a host component.
+fn validateProxyUrl(url: []const u8) !void {
+    // Must have a recognized scheme
+    var has_scheme = false;
+    for (valid_schemes) |scheme| {
+        if (std.mem.startsWith(u8, url, scheme)) {
+            has_scheme = true;
+            break;
+        }
+    }
+    if (!has_scheme) return error.InvalidUrl;
+
+    // Must parse as a valid URI with a host
+    const uri = std.Uri.parse(url) catch return error.InvalidUrl;
+    if (uri.host == null) return error.InvalidUrl;
+}
 
 /// Set or display the HTTP/HTTPS proxy.
 /// "default" clears the proxy (auto-detect from env vars).
@@ -16,13 +37,23 @@ pub fn run(
     stdout: *std.Io.Writer,
     stderr: *std.Io.Writer,
 ) !void {
-    _ = stderr;
-
     if (url) |u| {
         if (std.mem.eql(u8, u, "default")) {
             try zvm.settings.setProxy(allocator, zvm.io, "");
             try stdout.print("Reset proxy to auto-detect (from environment).\n", .{});
         } else {
+            validateProxyUrl(u) catch {
+                try terminal.printError(stderr, "Invalid proxy URL");
+                try stderr.print(
+                    \\Supported formats:
+                    \\  http://host:port
+                    \\  https://host:port
+                    \\  socks5://host:port
+                    \\
+                , .{});
+                try stderr.flush();
+                return;
+            };
             try zvm.settings.setProxy(allocator, zvm.io, u);
             try stdout.print("Set proxy to {s}\n", .{u});
         }