feat(fusion_clock): Web NFC integration on kiosk page

Adds NDEFReader scan loop, onNfcReading tap dispatcher, handleTap
state machine, postJson helper, capturePhoto stub (Task 17), and
setup wizard activation with error display.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

fusion_clock/static/src/js/fusion_clock_nfc_kiosk.js

@@ -112,20 +112,99 @@
     // ──────────────────────────────────────────────────────────────
     // Setup wizard
     // ──────────────────────────────────────────────────────────────
+    // ──────────────────────────────────────────────────────────────
+    // Web NFC reader
+    // ──────────────────────────────────────────────────────────────
+    let ndefReader = null;
+    let nfcReady = false;
+
+    async function startNfcReader() {
+        if (!("NDEFReader" in window)) {
+            throw new Error("Web NFC not supported on this browser/device. Use Chrome on Android.");
+        }
+        ndefReader = new NDEFReader();
+        await ndefReader.scan();
+        ndefReader.addEventListener("reading", onNfcReading);
+        ndefReader.addEventListener("readingerror", () => {
+            console.warn("[nfc-kiosk] reading error; reader still active");
+        });
+        nfcReady = true;
+    }
+
+    function onNfcReading(event) {
+        // event.serialNumber is the card UID — works for raw MIFARE access cards
+        const uid = (event.serialNumber || "").toUpperCase();
+        if (!uid) return;
+        if (currentState === STATE.ENROLL) {
+            // Enroll Mode handles taps differently (wired up in Task 18)
+            window.__nfcKiosk._onEnrollTap && window.__nfcKiosk._onEnrollTap(uid);
+            return;
+        }
+        if (currentState !== STATE.IDLE) return; // ignore taps mid-result
+        handleTap(uid);
+    }
+
+    async function handleTap(uid) {
+        setState(STATE.PROCESSING);
+        let photoB64 = "";
+        try {
+            photoB64 = await capturePhoto();
+        } catch (e) {
+            console.warn("[nfc-kiosk] camera capture failed", e);
+            // Server enforces photo_required if needed
+        }
+        try {
+            const result = await postJson("/fusion_clock/kiosk/nfc/tap", { card_uid: uid, photo_b64: photoB64 });
+            if (result.error === "debounce") {
+                // silent — back to IDLE
+                setState(STATE.IDLE);
+                return;
+            }
+            setState(STATE.RESULT, result);
+        } catch (e) {
+            setState(STATE.RESULT, { error: "network", message: "No connection. Please try again." });
+        }
+    }
+
+    async function postJson(url, params) {
+        const res = await fetch(url, {
+            method: "POST",
+            headers: { "Content-Type": "application/json" },
+            body: JSON.stringify({ jsonrpc: "2.0", method: "call", params }),
+        });
+        const json = await res.json();
+        return json.result || {};
+    }
+
+    // ──────────────────────────────────────────────────────────────
+    // Camera capture (real implementation in Task 17, stub for now)
+    // ──────────────────────────────────────────────────────────────
+    async function capturePhoto() {
+        return ""; // overridden in Task 17
+    }
+
+    // ──────────────────────────────────────────────────────────────
+    // Setup wizard activation
+    // ──────────────────────────────────────────────────────────────
     const setupBtn = document.getElementById("nfc_setup_start");
     if (setupBtn) {
         setupBtn.addEventListener("click", async () => {
-            // Web NFC + camera activation lives in Tasks 16 + 17
-            setState(STATE.IDLE);
+            try {
+                await startNfcReader();
+                setState(STATE.IDLE);
+            } catch (e) {
+                stateContainer.innerHTML = `
+                    <div class="nfc-kiosk__setup">
+                        <h2 style="color:#d9374e">Setup failed</h2>
+                        <p>${escapeHtml(e.message)}</p>
+                    </div>
+                `;
+            }
         });
     }
 
-    // Expose a tiny API for later tasks
     window.__nfcKiosk = {
-        setState,
-        STATE,
-        photoRequired,
-        debugEnabled,
-        locationConfigured,
+        setState, STATE, photoRequired, debugEnabled, locationConfigured,
+        handleTap, // exposed for mock-tap debug (Task 19)
     };
 })();