fix(dm): surface dm.name on chat header/placeholder; collapse unnamed-group placeholder

Three layered bugs all manifesting as "the group name doesn't show / the
placeholder is a 40-character wall of names":

1. **WS ready payload was missing `name`/`icon`.** The handler serialized
   DmChannel rows with only `id, federatedId, ownerId, createdAt, members,
   lastMessage`. The optional metadata fields were silently dropped, so
   the client store never received `dm.name` until a subsequent
   `dm_channel_updated` event fired (i.e. only mid-session renames worked,
   never the initial render). `ownerHomeUserId`, `ownerHomeInstance`, and
   `metadataUpdatedAt` were also missing — added too because federated
   routing depends on `ownerHomeInstance` (`getDmOwnerHomeInstance`).

2. **Header surfaces silently dropped `dm.name`.** `MainContent` (desktop
   chat header) and `MobileChatScreen` always rendered the joined member
   names, even when `dm.name` was set. Five other surfaces (`DmListItem`,
   `MobileDmsScreen`, `MessageList` welcome hero, `MobileGroupDmInfo`,
   `GroupDmSettings`) honored it correctly, so a renamed group showed
   different titles depending on which surface you looked at.

3. **Message-input placeholder rendered joined names.** Once a group
   has 4+ members "Message #Alice, Bob, Charlie, Dave" overflows the
   textarea and obscures the call-to-action.

Consolidates the display-name logic behind two utilities in
`dmFormatters.ts`:

  - `formatDmHeaderName(dm, currentUser)` — `dm.name` verbatim if set,
    else joined names (excluding self); falls back to `'Group'` /
    `'Direct Message'`. Used by all 5 header surfaces (was inlined
    5 different ways).
  - `formatDmInputLabel(dm, currentUser)` — `'#<name>'` if set,
    `'the group'` for unnamed groups (collapses the unreadable
    joined-names form), `'@<partner>'` for 1-on-1.

`MessageInput` accepts an optional `placeholder` prop that bypasses the
default `Message {#|@}<channelName>` derivation; DM call sites use it
to inject the `formatDmInputLabel`-based form. 1-on-1 DMs keep the
canonical-view lookup so replicated aliases still surface the home
account's displayName; the placeholder reuses the canonical `dmName`
so header + placeholder stay aligned even when raw partner ≠ canonical.

13 new unit tests covering `formatDmHeaderName` (8 cases: named, blank,
joined, federated-username base, empty group, 1-on-1, no-displayName,
no-partner) and `formatDmInputLabel` (4 cases: named, unnamed,
whitespace-only, 1-on-1). 365 → 377 web tests, 1053 server tests,
typecheck clean.

packages/web/src/components/chat/MessageInput.tsx

@@ -19,6 +19,15 @@ import { useVisualViewportInset } from '../../hooks/useVisualViewportInset';
 interface MessageInputProps {
   channelId: string;
   channelName: string;
+  /**
+   * Optional override for the textarea placeholder. When omitted the
+   * placeholder is derived from `channelName` (`'Message @user'` for DMs,
+   * `'Message #channel'` otherwise). DM call sites pass the resolved
+   * placeholder directly so they can collapse the unreadable joined-names
+   * form ("Message #Alice, Bob, Charlie, Dave") to "Message the group"
+   * when the group has no `dm.name` set.
+   */
+  placeholder?: string;
 }
 
 interface MentionState {
@@ -34,7 +43,7 @@ function makeFileHandleKey(): string {
   return `up-${Date.now()}-${Math.random().toString(36).slice(2, 10)}`;
 }
 
-export function MessageInput({ channelId, channelName }: MessageInputProps) {
+export function MessageInput({ channelId, channelName, placeholder }: MessageInputProps) {
   // Composer state lives in composerStore (per-channel, persisted)
   const composerState = useComposerStore((s) => s.states.get(channelId)) ?? {
     draftText: '',
@@ -889,7 +898,7 @@ export function MessageInput({ channelId, channelName }: MessageInputProps) {
             onChange={handleChange}
             onKeyDown={handleKeyDown}
             onPaste={canAttachFiles ? handlePaste : undefined}
-            placeholder={`Message ${channelName.startsWith('@') ? channelName : `#${channelName}`}`}
+            placeholder={placeholder ?? `Message ${channelName.startsWith('@') ? channelName : `#${channelName}`}`}
             className="input-embedded flex-1 py-[10px] px-1 resize-none text-[15px] leading-[1.375rem] max-h-[50vh] scrollbar-thin"
             rows={1}
           />