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/awaitable-websocket.js

@@ -18,12 +18,36 @@ class AwaitableWebSocket extends WebSocket {
     }
     this.onclose = e => {
       if (!this.#opened) {
-        reject(new Error(`WebSocket ${this.url} failed to connect, code ${e.code}`))
+        reject(new Error(`Failed to connect to server (code ${e.code})`))
         return
       }
-      this.#err = e.wasClean
-        ? new Error(`Websocket ${this.url} closed ${e.code}`)
-        : new Error(`WebSocket ${this.url} closed with error ${e.code}`)
+      // Create user-friendly close messages
+      let message
+      if (e.wasClean) {
+        // Standard close codes
+        switch (e.code) {
+          case 1000: message = 'Connection closed normally'; break
+          case 1001: message = 'Server is going away'; break
+          case 1002: message = 'Protocol error'; break
+          case 1003: message = 'Unsupported data received'; break
+          case 1006: message = 'Connection lost unexpectedly'; break
+          case 1007: message = 'Invalid data received'; break
+          case 1008: message = 'Policy violation'; break
+          case 1009: message = 'Message too large'; break
+          case 1010: message = 'Extension negotiation failed'; break
+          case 1011: message = 'Server encountered an error'; break
+          case 1012: message = 'Server is restarting'; break
+          case 1013: message = 'Server is overloaded, try again later'; break
+          case 1014: message = 'Bad gateway'; break
+          case 1015: message = 'TLS handshake failed'; break
+          default: message = `Connection closed (code ${e.code})`
+        }
+      } else {
+        message = e.code === 1006
+          ? 'Connection lost unexpectedly'
+          : `Connection closed with error (code ${e.code})`
+      }
+      this.#err = new Error(message)
       this.#waiting.splice(0).forEach(p => p.reject(this.#err))
     }
   }