refactor: improve donation tabs code organization

- Extract logic into dedicated functions for better readability
- Create handleDonationIntervalChange() as main click handler
- Separate concerns: updateLinkInterval, updateAmountDisplay, switchActiveTab
- Remove unnecessary comments in favor of clear function names
- Update footer to link script.js instead of donation-tabs.js

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

assets/js/script.js

@@ -0,0 +1,51 @@
+function updateLinkInterval(link, interval) {
+  const currentHref = link.getAttribute("href");
+  const newHref = currentHref.replace(
+    /default_interval=[om]/,
+    `default_interval=${interval}`
+  );
+  link.setAttribute("href", newHref);
+}
+
+function updateAmountDisplay(link, amount, interval) {
+  const boldText = link.querySelector(".bold");
+  if (!boldText) return;
+
+  const suffix = interval === "m" ? "€/mois" : "€";
+  boldText.innerHTML = `<span class="amount-value">${amount}</span>${suffix}`;
+}
+
+function switchActiveTab(clickedButton, allButtons) {
+  allButtons.forEach((btn) => btn.classList.remove("is-selected"));
+  clickedButton.classList.add("is-selected");
+}
+
+function handleDonationIntervalChange(interval) {
+  const donationLinks = document.querySelectorAll(".btn--donation[data-amount]");
+  const freeAmountLink = document.querySelector(".btn--donation:not([data-amount])");
+
+  donationLinks.forEach((link) => {
+    const amount = link.getAttribute("data-amount");
+    updateLinkInterval(link, interval);
+    updateAmountDisplay(link, amount, interval);
+  });
+
+  if (freeAmountLink) {
+    updateLinkInterval(freeAmountLink, interval);
+  }
+}
+
+document.addEventListener("DOMContentLoaded", function () {
+  const tabButtons = document.querySelectorAll(".nav--tabs__btn");
+  const donationLinks = document.querySelectorAll(".btn--donation[data-amount]");
+
+  if (tabButtons.length === 0 || donationLinks.length === 0) return;
+
+  tabButtons.forEach((button) => {
+    button.addEventListener("click", function () {
+      const interval = this.getAttribute("data-interval");
+      switchActiveTab(this, tabButtons);
+      handleDonationIntervalChange(interval);
+    });
+  });
+});