feat(federation): record DM origin alternatives in spaceStore

Every DM arriving in a ready payload with a federatedId now gets its
(origin, localChannelId) pair recorded in dmAlternatives, regardless of
whether the dedup pass kept this copy in dmChannels. Enables client-side
DM origin failover: when the primary origin drops, we can look up an
alternate origin's local channel ID for the same federated DM.

Prep for #10 (DM origin failover on disconnect).

packages/web/src/stores/spaceStore.ts

@@ -41,6 +41,8 @@ interface SpaceState {
   channelOriginMap: Map<string, string>; // channelId → instance origin ('' = home)
   voiceChannelIds: Set<string>; // channelIds that are voice channels (excluded from unread)
   categoryOriginMap: Map<string, string>; // categoryId → instance origin ('' = home)
+  /** federatedId → (origin → localChannelId). Every DM from every origin's ready payload is recorded here regardless of dedup outcome, so failover can re-point to an alternate origin's local channel ID. */
+  dmAlternatives: Map<string, Map<string, string>>;
   loadingSpaceId: string | null; // non-null while loadSpaceDetail is fetching
   _layoutUpdatedAt: number;
   setSpaces: (spaces: TaggedSpace[]) => void;
@@ -132,6 +134,7 @@ export const useSpaceStore = create<SpaceState>((set, get) => ({
   channelOriginMap: new Map(),
   voiceChannelIds: new Set(),
   categoryOriginMap: new Map(),
+  dmAlternatives: new Map(),
   loadingSpaceId: null,
   _layoutUpdatedAt: 0,
 
@@ -154,6 +157,7 @@ export const useSpaceStore = create<SpaceState>((set, get) => ({
       channelOriginMap: new Map(),
       voiceChannelIds: new Set(),
       categoryOriginMap: new Map(),
+      dmAlternatives: new Map(),
       loadingSpaceId: null,
       _layoutUpdatedAt: 0,
     });
@@ -584,6 +588,10 @@ export const useSpaceStore = create<SpaceState>((set, get) => ({
     const channelOriginMap = new Map(get().channelOriginMap);
     const voiceChannelIds = new Set(get().voiceChannelIds);
     const categoryOriginMap = new Map(get().categoryOriginMap);
+    const dmAlternatives = new Map<string, Map<string, string>>();
+    for (const [fid, byOrigin] of get().dmAlternatives) {
+      dmAlternatives.set(fid, new Map(byOrigin));
+    }
 
     // If home, clear home-origin entries first to avoid stale data
     if (isHome) {
@@ -688,6 +696,18 @@ export const useSpaceStore = create<SpaceState>((set, get) => ({
       }
     }
 
+    // Record every DM's (origin → localChannelId) for failover lookup,
+    // regardless of whether the dedup pass kept this copy in dmChannels.
+    for (const dm of incomingDms) {
+      if (!dm.federatedId) continue;
+      let byOrigin = dmAlternatives.get(dm.federatedId);
+      if (!byOrigin) {
+        byOrigin = new Map();
+        dmAlternatives.set(dm.federatedId, byOrigin);
+      }
+      byOrigin.set(origin, dm.id);
+    }
+
     // Merge: remove DMs belonging to this origin from existing state, then append incoming
     const existingDmsFromOtherOrigins = get().dmChannels.filter(dm => {
       const dmOrigin = get().channelOriginMap.get(dm.id);
@@ -711,6 +731,7 @@ export const useSpaceStore = create<SpaceState>((set, get) => ({
       channelOriginMap,
       voiceChannelIds,
       categoryOriginMap,
+      dmAlternatives,
     };
 
     // LWW layout merge: accept incoming layout only if its timestamp is >= ours