Migrate to new @welshman/domain + instance-based @welshman/app API

Adopts the rewritten welshman API: the removed @welshman/util helpers
(Profile/List/Room/Handler/Encryptable) are now Reader/Builder classes in
@welshman/domain, and @welshman/app dropped its global singletons for an App
instance + app.use(Plugin) registry.

- src/app/welshman.ts is now the app bootstrap + session-state module (one shared
  App instance, multi-account sessions/login, app-wide reactive views) rather than
  a compat shim re-exporting the old globals.
- Rewrote ~100 callers to use app.use(Plugin) directly (thunks, profiles, relays,
  rooms, zaps, tags, wot, feeds, sync); thunk helpers are now thunk methods.
- Added @welshman/domain dependency.
- Resolved residual gaps (storage hydration via plugin.onItem/wrapManager/Plaintext,
  relay-list mutators, search-relay list, outbox #d filter).

Best-effort: no toolchain/linking available, so this is not build- or
type-checked. Remaining judgment calls are flagged with TODO(welshman-migration).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BsMjvv7krpZeHK1Njeneru

src/app/components/ChatStart.svelte

@@ -4,7 +4,8 @@
   import {writable} from "svelte/store"
   import {tryCatch, uniq} from "@welshman/lib"
   import {fromNostrURI} from "@welshman/util"
-  import {loadMessagingRelayList} from "@welshman/app"
+  import {app} from "@app/welshman"
+  import {MessagingRelayLists} from "@welshman/app"
   import {preventDefault} from "@lib/html"
   import Field from "@lib/components/Field.svelte"
   import Button from "@lib/components/Button.svelte"
@@ -34,7 +35,7 @@
   let pubkeys: string[] = $state([])
 
   $effect(() => {
-    pubkeys.forEach(pubkey => loadMessagingRelayList(pubkey))
+    pubkeys.forEach(pubkey => app.use(MessagingRelayLists).load(pubkey))
   })
 
   onMount(() => {