Allow check intervals as low as 10 seconds

- Switch from Chrome alarms (30s min) to setTimeout loops
- Config now uses checkIntervalSeconds instead of minutes
- Default: 15 seconds, minimum: 10 seconds
- Prevents overlapping checks with isChecking flag
- Updated popup UI to show seconds

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

chrome-extension/popup.js

@@ -9,7 +9,7 @@ document.addEventListener("DOMContentLoaded", async () => {
   document.getElementById("webhook").value = config.discordWebhook || "";
   document.getElementById("urls").value = (config.urls || []).join("\n");
   document.getElementById("keywords").value = (config.keywords || []).join(", ");
-  document.getElementById("interval").value = config.checkIntervalMinutes || 1;
+  document.getElementById("interval").value = config.checkIntervalSeconds || 15;
   document.getElementById("enabled").checked = config.enabled !== false;
   document.getElementById("notifyNew").checked = config.notifyNewProducts !== false;
   document.getElementById("notifyRestock").checked = config.notifyRestocks !== false;
@@ -27,7 +27,7 @@ document.addEventListener("DOMContentLoaded", async () => {
       discordWebhook: document.getElementById("webhook").value.trim(),
       urls: urlsText.split("\n").map(u => u.trim()).filter(u => u),
       keywords: keywordsText.split(",").map(k => k.trim()).filter(k => k),
-      checkIntervalMinutes: parseInt(document.getElementById("interval").value) || 1,
+      checkIntervalSeconds: parseInt(document.getElementById("interval").value) || 15,
       enabled: document.getElementById("enabled").checked,
       notifyNewProducts: document.getElementById("notifyNew").checked,
       notifyRestocks: document.getElementById("notifyRestock").checked