Web: allow browser caching of static HTML pages

The login, app, and stats HTML pages are static assets compiled into
PROGMEM.  All dynamic content is fetched separately via JavaScript API
calls after the page loads — the HTML itself never changes between
requests within the same firmware version.

The "Cache-Control: no-store" header forced the browser to re-download
the full HTML on every visit, including a complete chunked transfer
from the ESP32.  Given that the ESP32 is slow and serves responses in
small chunks, this was an unnecessary repeated cost that added latency
before the page became interactive.

Removing no-store allows the browser to cache the HTML pages locally.
Subsequent requests are served from the browser cache instantly, with no
transfer from the ESP32 at all, leaving the server free to handle the
API requests that actually carry dynamic data.

src/helpers/web/WebPanelServer.cpp

@@ -2682,7 +2682,6 @@ esp_err_t WebPanelServer::handleIndex(httpd_req_t* req) {
   }
   ctx->self->noteActivity();
   httpd_resp_set_type(req, "text/html; charset=utf-8");
-  httpd_resp_set_hdr(req, "Cache-Control", "no-store");
   return sendProgmemChunked(req, kWebPanelLoginHtml);
 }
 
@@ -2709,7 +2708,6 @@ esp_err_t WebPanelServer::handleApp(httpd_req_t* req) {
   }
   ctx->self->noteActivity();
   httpd_resp_set_type(req, "text/html; charset=utf-8");
-  httpd_resp_set_hdr(req, "Cache-Control", "no-store");
   return sendProgmemChunked(req, kWebPanelAppHtml);
 }
 
@@ -2720,7 +2718,6 @@ esp_err_t WebPanelServer::handleStatsPage(httpd_req_t* req) {
   }
   ctx->self->noteActivity();
   httpd_resp_set_type(req, "text/html; charset=utf-8");
-  httpd_resp_set_hdr(req, "Cache-Control", "no-store");
   if (ctx->self->_runner != nullptr && !ctx->self->_runner->isWebStatsEnabled()) {
     return sendProgmemChunked(req, kWebPanelStatsDisabledHtml);
   }