Web: use sizeof instead of strlen for PROGMEM HTML page sizes

The HTML pages (login, app, stats) are string literals compiled into
PROGMEM. Their sizes are known at compile time, but the previous
sendProgmemChunked() called strlen() on every request to determine
the length, requiring a full scan of potentially tens of kilobytes of
PROGMEM data before the first byte was sent.

Replace sendProgmemChunked() with a sendWhole() function that accepts
an explicit length parameter, and introduce a sendProgmem() macro that
passes sizeof(mem) - 1 at the call site. For array literals this
resolves to a compile-time constant, eliminating the runtime strlen
scan entirely.

The rename from sendProgmemChunked() to sendWhole() also better reflects
what the function does — it sends the entire buffer in chunks — rather
than where the data comes from.

src/helpers/web/WebPanelServer.cpp

@@ -65,12 +65,13 @@ esp_err_t sendChunk(httpd_req_t* req, const char* text) {
   return httpd_resp_sendstr_chunk(req, text != nullptr ? text : "");
 }
 
-esp_err_t sendProgmemChunked(httpd_req_t* req, const char* text) {
+#define sendProgmem(req, mem)  sendWhole(req, mem, sizeof(mem) - 1)
+
+esp_err_t sendWhole(httpd_req_t* req, const char* text, const size_t len) {
   if (text == nullptr) {
     return httpd_resp_send_chunk(req, nullptr, 0);
   }
 
-  const size_t len = strlen(text);
   size_t offset = 0;
   while (offset < len) {
     const size_t chunk_len = ((len - offset) > kWebPageChunkSize) ? kWebPageChunkSize : (len - offset);
@@ -2682,7 +2683,7 @@ esp_err_t WebPanelServer::handleIndex(httpd_req_t* req) {
   }
   ctx->self->noteActivity();
   httpd_resp_set_type(req, "text/html; charset=utf-8");
-  return sendProgmemChunked(req, kWebPanelLoginHtml);
+  return sendProgmem(req, kWebPanelLoginHtml);
 }
 
 esp_err_t WebPanelServer::handleHttpRedirect(httpd_req_t* req) {
@@ -2708,7 +2709,7 @@ esp_err_t WebPanelServer::handleApp(httpd_req_t* req) {
   }
   ctx->self->noteActivity();
   httpd_resp_set_type(req, "text/html; charset=utf-8");
-  return sendProgmemChunked(req, kWebPanelAppHtml);
+  return sendProgmem(req, kWebPanelAppHtml);
 }
 
 esp_err_t WebPanelServer::handleStatsPage(httpd_req_t* req) {
@@ -2719,9 +2720,9 @@ esp_err_t WebPanelServer::handleStatsPage(httpd_req_t* req) {
   ctx->self->noteActivity();
   httpd_resp_set_type(req, "text/html; charset=utf-8");
   if (ctx->self->_runner != nullptr && !ctx->self->_runner->isWebStatsEnabled()) {
-    return sendProgmemChunked(req, kWebPanelStatsDisabledHtml);
+    return sendProgmem(req, kWebPanelStatsDisabledHtml);
   }
-  return sendProgmemChunked(req, kWebPanelAppHtml);
+  return sendProgmem(req, kWebPanelAppHtml);
 }
 
 esp_err_t WebPanelServer::handleLogin(httpd_req_t* req) {