Implement code word based remote authentication (#1)

Add comprehensive remote authentication system allowing users to log in from one device by authenticating from another trusted device. Features include:

- Proof of Work (PoW) protection using PBKDF2-SHA512 to prevent abuse
- Simple pairing codes (3 words) protected by dynamic PoW difficulty
- Autocomplete pairing code input with error checking
- Real-time WebSocket communication between devices

Unlike device addition links and reset links with QR codes that only allow adding an authentication method, and that work offline over the duration of several days, this mechanism is strictly online, with 5 minute time limit.

frontend/src/utils/base64url.js

@@ -0,0 +1,33 @@
+/**
+ * URL-safe Base64 encoding/decoding utilities.
+ *
+ * These functions handle base64url format (RFC 4648) which uses:
+ * - '-' instead of '+'
+ * - '_' instead of '/'
+ * - No padding '=' characters
+ */
+
+/**
+ * Decode a base64url string to Uint8Array.
+ * Handles both standard base64 and URL-safe base64 (with or without padding).
+ * @param {string} str - Base64url encoded string
+ * @returns {Uint8Array} - Decoded bytes
+ */
+export function dec(str) {
+  // Convert URL-safe characters to standard base64
+  const base64 = str.replace(/-/g, '+').replace(/_/g, '/')
+  // Add padding if needed
+  const padded = base64 + '='.repeat((4 - base64.length % 4) % 4)
+  return Uint8Array.from(atob(padded), c => c.charCodeAt(0))
+}
+
+/**
+ * Encode a Uint8Array to base64url string.
+ * @param {Uint8Array} bytes - Bytes to encode
+ * @returns {string} - Base64url encoded string (no padding)
+ */
+export function enc(bytes) {
+  const base64 = btoa(String.fromCharCode(...bytes))
+  // Convert to URL-safe and remove padding
+  return base64.replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '')
+}