feat: guard the per-path EQ-rule store against the NVS 4 KB limit

Per-path EQ rules are serialized into a single NVS string under "eqRules".
NVS caps a string value at 4000 bytes, so once enough rules were added the
putString() silently failed (returned 0) and the rule was lost without any
feedback.

Reject an over-full save up front with HTTP 413 (body
{"error":"eqRulesFull","used":N,"max":N}) while leaving the previously stored
rules untouched, and surface a failed NVS write as 500. The Files tab now uses
fetch() for setEqRule so it can show a clear "EQ rule storage is full" toast
instead of dropping the rule silently. i18n de/en/fr, Swagger updated.

Verified on device: adding rules until the cap returned
413 {"error":"eqRulesFull","used":3962,"max":3900}; the store stayed at
3853 bytes (under the limit) and the previously stored rules were intact.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: fhirschmann

README.md

@@ -101,6 +101,7 @@ neon logo that doubles as the SVG favicon ([`7be5254`](../../commit/7be5254)):
 
 | Change | Commit |
 | --- | --- |
+| **EQ-rule store overflow guard**: per-path EQ rules are serialized into a single NVS string capped at ~4 KB; adding too many used to silently fail. The device now rejects an over-full save with `HTTP 413` (and reports a failed NVS write as `500`), and the Files tab surfaces a clear *"EQ rule storage is full"* toast instead of losing the rule without feedback | [`PENDING5`](../../commit/PENDING5) |
 | **HTTP-sync robustness**: the file-sync manifest is now streamed straight from the network into the JSON parser (instead of buffering the whole payload in a `String` first), roughly halving peak RAM during the parse on large manifests; the cross-core sync progress message is guarded by a spinlock so the web UI can never read a half-written line | [`PENDING3`](../../commit/PENDING3) |
 | PWA support: web app manifest + app icon, "add to home screen" with proper icon and name | [`b4287b9`](../../commit/b4287b9) |
 | PWA offline fallback: a service worker serves a cyberpunk "ESPuino Offline" page (with auto-reconnect) instead of a black screen when the home-screen app is launched while the player is powered off | [`bd07a7c`](../../commit/bd07a7c) |

REST-API.yaml

@@ -810,6 +810,10 @@ paths:
       responses:
         "200":
           description: Rule stored.
+        "413":
+          description: "Rule store is full (NVS ~4 KB limit). Body: {\"error\":\"eqRulesFull\",\"used\":N,\"max\":N}. Delete existing rules first."
+        "500":
+          description: Saving to NVS failed.
     delete:
       tags: [Fork]
       summary: Delete a per-path equalizer rule.

html/locales/de.json

@@ -666,7 +666,9 @@
 		"eqrules": {
 			"title": "EQ-Regeln (pro Datei/Ordner)",
 			"desc": "Übersicht der per-Pfad zugewiesenen Equalizer-Profile. Hier kannst du sie ansehen und löschen.",
-			"empty": "Keine EQ-Regeln gesetzt."
+			"empty": "Keine EQ-Regeln gesetzt.",
+			"full": "Der Speicher für EQ-Regeln ist voll — lösche bestehende Regeln, bevor du neue hinzufügst.",
+			"savefailed": "EQ-Regel konnte nicht gespeichert werden."
 		},
 		"backup": {
 			"title": "Backup",

html/locales/en.json

@@ -666,7 +666,9 @@
 		"eqrules": {
 			"title": "EQ rules (per file/folder)",
 			"desc": "Overview of the per-path equalizer profiles. View and delete them here.",
-			"empty": "No EQ rules set."
+			"empty": "No EQ rules set.",
+			"full": "EQ rule storage is full — delete some existing rules before adding new ones.",
+			"savefailed": "Could not save the EQ rule."
 		},
 		"backup": {
 			"title": "Backup",

html/locales/fr.json

@@ -655,7 +655,9 @@
 		"eqrules": {
 			"title": "Règles EQ (par fichier/dossier)",
 			"desc": "Aperçu des profils d'égaliseur attribués par chemin. Vous pouvez les consulter et les supprimer ici.",
-			"empty": "Aucune règle EQ définie."
+			"empty": "Aucune règle EQ définie.",
+			"full": "Le stockage des règles EQ est plein — supprimez des règles existantes avant d'en ajouter.",
+			"savefailed": "Impossible d'enregistrer la règle EQ."
 		},
 		"backup": {
 			"title": "Sauvegarde",

html/management.html

@@ -6845,9 +6845,25 @@ <h5 class="modal-title" data-i18n="tools.nvs.erase.title"></h5>
 		function setEqRule(path, profileKey) {
 			const g = eqProfiles[profileKey];
 			if (!g) return;
-			postData("http://" + host + "/eqrule?path=" + encodeURIComponent(path)
+			// fetch (not postData) so we can surface the server's error: NVS caps the rule
+			// store at ~4 KB and the device rejects an over-full save with HTTP 413 instead
+			// of silently dropping the rule.
+			fetch("http://" + host + "/eqrule?path=" + encodeURIComponent(path)
 				+ "&low=" + g.gainLowPass + "&band=" + g.gainBandPass + "&high=" + g.gainHighPass
-				+ "&profile=" + encodeURIComponent(profileKey), function () { loadEqRules(function () { updateEqIndicator(); updateSelectedNodeBadges(); }); });
+				+ "&profile=" + encodeURIComponent(profileKey), { method: "POST" })
+				.then(function (resp) {
+					if (resp.status === 413) {
+						toaster.error(i18next.t("tools.eqrules.full"));
+						return;
+					}
+					if (!resp.ok) {
+						toaster.error(i18next.t("tools.eqrules.savefailed"));
+						return;
+					}
+					loadEqRules(function () { updateEqIndicator(); updateSelectedNodeBadges(); });
+					flashActionOk();
+				})
+				.catch(function () { toaster.error(i18next.t("tools.eqrules.savefailed")); });
 		}
 		// Remove the per-path EQ rule for a file/folder path.
 		function deleteEqRule(path) {

src/Web.cpp

@@ -2143,6 +2143,10 @@ void handleGetSettings(AsyncWebServerRequest *request) {
 	request->send(response);
 }
 
+// NVS caps a single string value at 4000 bytes; stay safely below that so a near-full
+// rule set still leaves room for the JSON envelope and a final entry.
+static constexpr size_t EQ_RULES_MAX_BYTES = 3900;
+
 // Return all per-path equalizer rules as the stored JSON array.
 void handleGetEqRules(AsyncWebServerRequest *request) {
 	request->send(200, "application/json", gPrefsSettings.getString("eqRules", "[]"));
@@ -2183,7 +2187,20 @@ void handleSetEqRule(AsyncWebServerRequest *request) {
 
 	String out;
 	serializeJson(doc, out);
-	gPrefsSettings.putString("eqRules", out);
+	// NVS caps a single string value at 4000 bytes. If the serialized rule set would
+	// exceed that, putString() silently fails (returns 0) and the rule is lost without
+	// any feedback. Reject the save up-front with a clear error instead, leaving the
+	// previously stored rules untouched, and tell the UI how full the store is.
+	if (out.length() > EQ_RULES_MAX_BYTES) {
+		char err[160];
+		snprintf(err, sizeof(err), "{\"error\":\"eqRulesFull\",\"used\":%u,\"max\":%u}", (unsigned) out.length(), (unsigned) EQ_RULES_MAX_BYTES);
+		request->send(413, "application/json", err);
+		return;
+	}
+	if (gPrefsSettings.putString("eqRules", out) == 0) {
+		request->send(500, "application/json", "{\"error\":\"eqRulesSaveFailed\"}");
+		return;
+	}
 	AudioPlayer_ReloadEqRules();
 	request->send(200, "application/json", "{}");
 }