Finish axios migration via shared native HTTP helper

Replace all in-scope axios callsites with a new helpers/http.js wrapper over native fetch, including JSON/text GET, JSON POST, HEAD checks, and transient 5xx retry behavior; update all browser, build, script, and proxy API clients to use it; add focused unit tests; and remove axios from package dependencies.

Constraint: Preserve API/build and deployment behavior while lowering transport surface area.

Rejected: inline fetch replacements at each callsite | rejected to avoid inconsistent error/retry semantics.

Confidence: high

Scope-risk: moderate

Directive: Keep helper in place as the transport boundary and update tests when changing request semantics.

Tested: pnpm run -s typecheck, pnpm -s run test-prebuild, pnpm -s run test, pnpm -s run test:browser, pnpm -s run netlify-build, smoke GETs on /apple-silicon-app-test and /apple-silicon-app-test/?version=2

Not-tested: branch/netlify deployment health in CI pipeline after merge

helpers/build-device-list.js

@@ -1,6 +1,5 @@
-import axios from 'axios'
-
 import { makeSlug } from './slug.js'
+import { getJson } from './http.js'
 
 export function getDeviceEndpoint ( slug ) {
     return `/device/${ slug }`
@@ -12,10 +11,7 @@ export default async function () {
 
     const devicesJsonUrl = `${process.env.VFUNCTIONS_URL}/api/devices`
 
-    const rawDeviceList = await axios.get(devicesJsonUrl)
-        .then( response => {
-            return response.data
-        })
+    const rawDeviceList = await getJson( devicesJsonUrl )
         .catch(function (error) {
             // handle error
             console.warn('Error fetching device list', error)