Improve clocks

There's a flaky performance test that I wanted to fix (1). This led to a couple
changes.

1 - Add timestamp() and milliTimestamp() to datetime.zig. Reduce some code
    duplication and use better clock_ids where available

2 - Change Performance API to use milliTimestamp and store a u64 instead of a
    f64. While the spec says a float, Firefox deals with u64 and implicit
    conversion is always available. Makes our APIs simpler.

(1) - https://github.com/lightpanda-io/browser/actions/runs/17313296490/job/49151366798#step:4:131

src/server.zig

@@ -897,8 +897,7 @@ fn buildJSONVersionResponse(
 }
 
 fn timestamp() u32 {
-    const ts = std.posix.clock_gettime(std.posix.CLOCK.MONOTONIC) catch unreachable;
-    return @intCast(ts.sec);
+    return @import("datetime.zig").timestamp();
 }
 
 // In-place string lowercase