chore: inline ga4 module and remove raw GitHub import (#3733)

## Summary
- Inlined the `$ga4` module (from
`https://raw.githubusercontent.com/denoland/ga4/main/mod.ts`) into
`www/utils/ga4.ts`
- Replaced old `deno.land/std` URL imports with `@std/http`
(`getCookies`, `STATUS_TEXT`)
- Inlined the trivial `snakeCase` helper (was from `deno.land/x/case`)
- Removed `$ga4` import map entry from `deno.json`

The ga4 package was never published to jsr or npm, so inlining was the
cleanest way to eliminate the last raw URL-based dependency.

Author: bartlomieju

deno.json

@@ -61,7 +61,6 @@
     "mime-db": "npm:mime-db@^1.54.0",
     "preact": "npm:preact@^10.28.2",
     "preact-render-to-string": "npm:preact-render-to-string@^6.6.5",
-    "$ga4": "https://raw.githubusercontent.com/denoland/ga4/main/mod.ts",
     "@opentelemetry/api": "npm:@opentelemetry/api@^1.9.0",
     "@preact/signals": "npm:@preact/signals@^2.5.1",
     "@std/encoding": "jsr:@std/encoding@1",

deno.lock

@@ -1,6 +1,6 @@
 import type { Context } from "fresh";
-import type { Event } from "$ga4";
-import { GA4Report, isDocument, isServerError } from "$ga4";
+import type { Event } from "../utils/ga4.ts";
+import { GA4Report, isDocument, isServerError } from "../utils/ga4.ts";
 
 const GA4_MEASUREMENT_ID = Deno.env.get("GA4_MEASUREMENT_ID");