fix(onboarding): don't fail first run when the relay forbids member channel creation

[mattbalza]

Problem

A new teammate installed Buzz against buzz.scalarly.org and hit a red error on first run:

403 Forbidden — restricted: only workspace owners may create durable channels or forums

That message is our relay working as configured (BUZZ_RELAY_CHANNEL_CREATE_POLICY=owner-only,
crates/buzz-relay/src/handlers/ingest.rs:2260). What's broken is the client's reaction to it.

Desktop's first run creates a personal, private "Welcome" channel per member. On an
owner-only relay that create is rejected for every non-owner, and:

1. initializeStarterChannels already wraps ensureStarterChannels in try/catch, but
   ensureWelcomeChannel threw straight through to the outer catch → {ok: false}.
2. CommunityOnboardingFlow.finalize() re-throws on !result.ok → "Couldn't set up
   starter channels", with "Take me to Buzz" only turning into "Skip for now" after two failures.
3. markWelcomeChannelEnsured never ran, so the mount retry effect re-attempted the
   rejected create forever — visible in the relay log as a 403 roughly every 46s.

The member was fully provisioned and every shared channel was reachable. The only thing
missing was an optional personal room.

Fix

Treat the private Welcome channel as a nicety the community may not offer:

• ensureOptionalWelcomeChannel (welcome.ts) resolves to
  {channel: null, unavailableReason} instead of rejecting.
• initializeStarterChannels skips the query-cache seeding, seedWelcomeExperience,
  rememberPendingWelcomeChannel and notifyWelcomeChannelReady when there is no channel,
  and calls markWelcomeChannelEnsured so the retry effect stops re-attempting.
• resolveWelcomeFocusChannelId lands first run on welcome-everyone, falling back to
  general, when there is no private Welcome channel.

Genuine starter-channel failures still return {ok: false} and surface exactly as before —
only the optional personal room is downgraded.

Test plan

• desktop/src/features/onboarding/welcome.test.mjs — 5 new cases: success passthrough,
  the owner-only rejection (using the relay's verbatim message), a non-Error rejection,
  and both focus-fallback rungs. Written failing first, then made green.
• Full desktop suite: 3884 passed / 0 failed.
• pnpm exec tsc --noEmit, pnpm exec biome lint src/features/onboarding, pnpm build — clean.
• Manual: fresh install against an owner-only relay reaches the app and lands in
  welcome-everyone with no error toast; against an owner/any-member relay the private
  Welcome channel is created and focused exactly as before.