setup: add ← Back option to Discord, WhatsApp, iMessage channel flows

Picking the wrong messaging channel during setup left users with no way
to bail out — they had to either complete the chosen flow or kill setup
and start over. This adds a Back option to the first prompt of three
channel sub-flows that share the same simple shape (one leading
brightSelect that's easy to extend).

Mechanics:
- New `setup/lib/back-nav.ts` exports a BACK_TO_CHANNEL_SELECTION
  sentinel and ChannelFlowResult type.
- `setup/auto.ts` wraps the channel dispatch in a while-loop; channels
  return BACK_TO_CHANNEL_SELECTION to bounce back to the chooser
  without restarting setup. Channels not yet wired return void and the
  loop exits after one pass, so the change is backwards compatible.
- Discord, WhatsApp, iMessage each add a `← Back to channel selection`
  option to their first prompt.

Telegram, Slack, Teams, and Signal will follow as separate PRs — they
each need a slightly different shape (extra prompt insertions, gating
inside multi-step flows, etc.) and are easier to review independently.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

setup/channels/whatsapp.ts

@@ -33,6 +33,7 @@ import * as p from '@clack/prompts';
 import k from 'kleur';
 
 import * as setupLog from '../logs.js';
+import { BACK_TO_CHANNEL_SELECTION, type ChannelFlowResult } from '../lib/back-nav.js';
 import { brightSelect } from '../lib/bright-select.js';
 import { getLaunchdLabel, getSystemdUnit } from '../../src/install-slug.js';
 import {
@@ -53,8 +54,9 @@ const AUTH_CREDS_PATH = path.join(process.cwd(), 'store', 'auth', 'creds.json');
 
 type AuthMethod = 'qr' | 'pairing-code';
 
-export async function runWhatsAppChannel(displayName: string): Promise<void> {
+export async function runWhatsAppChannel(displayName: string): Promise<ChannelFlowResult> {
   const method = await askAuthMethod();
+  if (method === 'back') return BACK_TO_CHANNEL_SELECTION;
   const phone = method === 'pairing-code' ? await askPhoneNumber() : undefined;
 
   const install = await runQuietChild(
@@ -148,7 +150,7 @@ export async function runWhatsAppChannel(displayName: string): Promise<void> {
   }
 }
 
-async function askAuthMethod(): Promise<AuthMethod> {
+async function askAuthMethod(): Promise<AuthMethod | 'back'> {
   const choice = ensureAnswer(
     await brightSelect({
       message: 'How would you like to authenticate with WhatsApp?',
@@ -163,10 +165,14 @@ async function askAuthMethod(): Promise<AuthMethod> {
           label: 'Enter a pairing code on your phone',
           hint: 'no camera needed',
         },
+        {
+          value: 'back',
+          label: '← Back to channel selection',
+        },
       ],
     }),
-  ) as AuthMethod;
-  setupLog.userInput('whatsapp_auth_method', choice);
+  ) as AuthMethod | 'back';
+  if (choice !== 'back') setupLog.userInput('whatsapp_auth_method', choice);
   return choice;
 }