drop _headers attempt; accept upstream's TTL on /resources/*

Cloudflare Pages serves /resources/config.js with max-age=14400
regardless of what _headers declares (verified live: cache-control
header unchanged 2h after deploy, cf-cache-status: MISS). Whatever's
producing the 4h TTL on Pages is not overridable from the deploy
artifact alone -- needs dashboard access we don't have.

Upstream mempool doesn't try to cache-bust /resources/config.js either.
Following that lead: drop the _headers file and its angular.json asset
registration, accept that already-open tabs see a stale
GIT_COMMIT_HASH in the footer until their cache expires (Finding 1
made this lower-stakes -- the footer link is the only thing affected;
isOtsCommit ships correctly via the proxy-injection path).

Author: hans-crypto

frontend/angular.json

@@ -173,7 +173,6 @@
               "src/customize.js",
               "src/config.template.js",
               "src/_redirects",
-              "src/_headers",
               {
                 "glob": "*.css",
                 "input": ".theme-build",
@@ -199,8 +198,7 @@
               "assets": [
                 "src/favicon.ico",
                 "src/robots.txt",
-                "src/_redirects",
-                "src/_headers"
+                "src/_redirects"
               ],
               "fileReplacements": [
                 {