diff --git a/frontend/src/App.tsx b/frontend/src/App.tsx
index 9e1627a..46c6000 100644
--- a/frontend/src/App.tsx
+++ b/frontend/src/App.tsx
@@ -28,9 +28,80 @@ function base64UrlEncode(buffer: ArrayBuffer) {
 }
 
 async function sha256(text: string) {
+  // Prefer SubtleCrypto when available (secure contexts). Fallback to a
+  // pure-JS implementation for environments where `crypto.subtle` is undefined.
   const data = new TextEncoder().encode(text)
-  const hash = await window.crypto.subtle.digest('SHA-256', data)
-  return base64UrlEncode(hash)
+  if (typeof crypto !== 'undefined' && crypto.subtle && typeof crypto.subtle.digest === 'function') {
+    const hash = await crypto.subtle.digest('SHA-256', data)
+    return base64UrlEncode(hash)
+  }
+
+  // JS fallback SHA-256 (returns ArrayBuffer)
+  function sha256Fallback(message: Uint8Array) {
+    const H = new Uint32Array([0x6a09e667, 0xbb67ae85, 0x3c6ef372, 0xa54ff53a, 0x510e527f, 0x9b05688c, 0x1f83d9ab, 0x5be0cd19])
+    const K = new Uint32Array([
+      0x428a2f98,0x71374491,0xb5c0fbcf,0xe9b5dba5,0x3956c25b,0x59f111f1,0x923f82a4,0xab1c5ed5,
+      0xd807aa98,0x12835b01,0x243185be,0x550c7dc3,0x72be5d74,0x80deb1fe,0x9bdc06a7,0xc19bf174,
+      0xe49b69c1,0xefbe4786,0x0fc19dc6,0x240ca1cc,0x2de92c6f,0x4a7484aa,0x5cb0a9dc,0x76f988da,
+      0x983e5152,0xa831c66d,0xb00327c8,0xbf597fc7,0xc6e00bf3,0xd5a79147,0x06ca6351,0x14292967,
+      0x27b70a85,0x2e1b2138,0x4d2c6dfc,0x53380d13,0x650a7354,0x766a0abb,0x81c2c92e,0x92722c85,
+      0xa2bfe8a1,0xa81a664b,0xc24b8b70,0xc76c51a3,0xd192e819,0xd6990624,0xf40e3585,0x106aa070,
+      0x19a4c116,0x1e376c08,0x2748774c,0x34b0bcb5,0x391c0cb3,0x4ed8aa4a,0x5b9cca4f,0x682e6ff3,
+      0x748f82ee,0x78a5636f,0x84c87814,0x8cc70208,0x90befffa,0xa4506ceb,0xbef9a3f7,0xc67178f2
+    ])
+
+    const ml = message.length * 8
+    const withOne = new Uint8Array(((message.length + 9 + 63) >> 6) << 6)
+    withOne.set(message)
+    withOne[message.length] = 0x80
+    const view = new DataView(withOne.buffer)
+    view.setUint32(withOne.length - 4, ml >>> 0)
+    view.setUint32(withOne.length - 8, Math.floor(ml / 0x100000000))
+
+    const w = new Uint32Array(64)
+    for (let i = 0; i < withOne.length; i += 64) {
+      for (let t = 0; t < 16; ++t) w[t] = view.getUint32(i + t * 4)
+      for (let t = 16; t < 64; ++t) {
+        const s0 = (rightRotate(w[t - 15], 7) ^ rightRotate(w[t - 15], 18) ^ (w[t - 15] >>> 3)) >>> 0
+        const s1 = (rightRotate(w[t - 2], 17) ^ rightRotate(w[t - 2], 19) ^ (w[t - 2] >>> 10)) >>> 0
+        w[t] = (w[t - 16] + s0 + w[t - 7] + s1) >>> 0
+      }
+
+      let a = H[0], b = H[1], c = H[2], d = H[3], e = H[4], f = H[5], g = H[6], h = H[7]
+      for (let t = 0; t < 64; ++t) {
+        const S1 = (rightRotate(e, 6) ^ rightRotate(e, 11) ^ rightRotate(e, 25)) >>> 0
+        const ch = ((e & f) ^ (~e & g)) >>> 0
+        const temp1 = (h + S1 + ch + K[t] + w[t]) >>> 0
+        const S0 = (rightRotate(a, 2) ^ rightRotate(a, 13) ^ rightRotate(a, 22)) >>> 0
+        const maj = ((a & b) ^ (a & c) ^ (b & c)) >>> 0
+        const temp2 = (S0 + maj) >>> 0
+
+        h = g; g = f; f = e; e = (d + temp1) >>> 0
+        d = c; c = b; b = a; a = (temp1 + temp2) >>> 0
+      }
+
+      H[0] = (H[0] + a) >>> 0
+      H[1] = (H[1] + b) >>> 0
+      H[2] = (H[2] + c) >>> 0
+      H[3] = (H[3] + d) >>> 0
+      H[4] = (H[4] + e) >>> 0
+      H[5] = (H[5] + f) >>> 0
+      H[6] = (H[6] + g) >>> 0
+      H[7] = (H[7] + h) >>> 0
+    }
+
+    const out = new Uint8Array(32)
+    const outView = new DataView(out.buffer)
+    for (let i = 0; i < 8; ++i) outView.setUint32(i * 4, H[i])
+    return out.buffer
+  }
+
+  function rightRotate(value: number, amount: number) {
+    return (value >>> amount) | (value << (32 - amount))
+  }
+
+  const hashBuf = sha256Fallback(data)
+  return base64UrlEncode(hashBuf)
 }
 
 async function createCodeVerifier() {