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/pagefind/load-sitemap-endpoints.ts

@@ -1,16 +1,13 @@
 import fs from 'fs-extra'
-import axios from 'axios'
+import {
+    getJson,
+    shouldRetryError
+} from '~/helpers/http.js'
 
 import {
     sitemapEndpointsPath
 } from '~/helpers/pagefind/config.js'
 
-function shouldRetryError ( error: unknown ) {
-    const status = ( error as { response?: { status?: number } } )?.response?.status
-
-    return typeof status === 'number' && status >= 500
-}
-
 async function fetchJsonWithRetries (
     url: string,
     {
@@ -21,25 +18,10 @@ async function fetchJsonWithRetries (
         delayMs?: number
     } = {}
 ) {
-    let lastError: unknown
-
-    for ( let attempt = 1; attempt <= attempts; attempt += 1 ) {
-        try {
-            const response = await axios.get( url )
-
-            return response.data
-        } catch ( error ) {
-            lastError = error
-
-            if ( attempt >= attempts || !shouldRetryError( error ) ) {
-                throw error
-            }
-
-            await new Promise( resolve => setTimeout( resolve, delayMs ) )
-        }
-    }
-
-    throw lastError
+    return await getJson( url, {
+        attempts,
+        delayMs
+    } )
 }
 
 export async function loadSitemapEndpoints () {