feat(cli): connect via relay using pairing offer URL (#639)

Pass `--host https://app.paseo.sh/#offer=...` (or `PASEO_HOST=...`) to
any CLI command and the connection routes through the relay with E2EE,
the same way the mobile and desktop apps connect. Same `paseo daemon
pair` link the app already accepts — no new pairing flow.

Closes #181

packages/server/src/shared/connection-offer.ts

@@ -1,3 +1,4 @@
+import { Buffer } from "node:buffer";
 import { z } from "zod";
 
 /**
@@ -19,3 +20,38 @@ export type ConnectionOfferV2 = z.infer<typeof ConnectionOfferV2Schema>;
 
 export const ConnectionOfferSchema = ConnectionOfferV2Schema;
 export type ConnectionOffer = ConnectionOfferV2;
+
+function decodeBase64UrlToUtf8(input: string): string {
+  const base64 = input.replace(/-/g, "+").replace(/_/g, "/");
+  const padded = base64.padEnd(base64.length + ((4 - (base64.length % 4)) % 4), "=");
+  return Buffer.from(padded, "base64").toString("utf8");
+}
+
+export function decodeOfferFragmentPayload(encoded: string): unknown {
+  const json = decodeBase64UrlToUtf8(encoded);
+  return JSON.parse(json) as unknown;
+}
+
+const OFFER_FRAGMENT_PREFIX = "#offer=";
+
+function extractOfferFragmentEncoded(input: string): string | null {
+  const trimmed = input.trim();
+  if (!trimmed) return null;
+  const fragmentIndex = trimmed.indexOf(OFFER_FRAGMENT_PREFIX);
+  if (fragmentIndex === -1) return null;
+  const encoded = trimmed.slice(fragmentIndex + OFFER_FRAGMENT_PREFIX.length).trim();
+  return encoded.length > 0 ? encoded : null;
+}
+
+/**
+ * Parse a pairing-offer URL of the form `https://app.paseo.sh/#offer=<base64url>`.
+ *
+ * Returns `null` if the input has no `#offer=` fragment. Throws if the fragment
+ * exists but the payload is malformed or fails schema validation.
+ */
+export function parseConnectionOfferFromUrl(input: string): ConnectionOffer | null {
+  const encoded = extractOfferFragmentEncoded(input);
+  if (!encoded) return null;
+  const payload = decodeOfferFragmentPayload(encoded);
+  return ConnectionOfferSchema.parse(payload);
+}