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/RelayList.svelte

@@ -1,7 +1,6 @@
 <script lang="ts">
   import type {Readable} from "svelte/store"
   import {SvelteSet} from "svelte/reactivity"
-  import {waitForThunkError} from "@welshman/app"
   import type {Thunk} from "@welshman/app"
   import CloseCircle from "@assets/icons/close-circle.svg?dataurl"
   import AltArrowLeft from "@assets/icons/alt-arrow-left.svg?dataurl"
@@ -37,7 +36,7 @@
     loading.add(url)
 
     try {
-      const error = await waitForThunkError(await removeRelay(url))
+      const error = await (await removeRelay(url)).waitForError()
 
       if (error) {
         pushToast({