feat: resource caps, Saved Messages, author walls, docs for node bring-up

Node flags (cmd/node/main.go):
  --max-cpu / --max-ram-mb — Go runtime caps (GOMAXPROCS / GOMEMLIMIT)
  --feed-disk-limit-mb — hard 507 refusal for new post bodies over quota
  --chain-disk-limit-mb — advisory watcher (can't reject blocks without
  breaking consensus; logs WARN every minute)

Client — Saved Messages (self-chat):
  - Auto-created on sign-in, pinned top of chat list, blue bookmark avatar
  - Send short-circuits the relay (no encrypt, no fee, no mailbox hop)
  - Empty state rendered outside inverted FlatList — fixes the mirrored
    "say hi…" on Android RTL-aware layout builds
  - PostCard shows "You" for own posts instead of the self-contact alias

Client — user walls:
  - New route /(app)/feed/author/[pub] with infinite-scroll via
    `created_at` cursor and pull-to-refresh
  - Profile screen gains "View posts" button (universal) next to
    "Open chat" (contact-only)

Feed pipeline:
  - Bump client JPEG quality 0.5 → 0.75 to match server scrubber (Q=75),
    so a 60 KiB compose doesn't balloon past 256 KiB after server re-encode
  - ErrPostTooLarge now wraps with the actual size vs cap, errors.Is
    preserved in the HTTP layer
  - FeedMailbox quota + DiskUsage surface — supports new CLI flag

README:
  - Step-by-step "first node / joiner" section on the landing page,
    full flag tables incl. the new resource-cap group, minimal
    checklists for open/private/low-end deployments

client-app/lib/api.ts

@@ -367,6 +367,39 @@ export interface IdentityInfo {
   registered: boolean;
 }
 
+/**
+ * Relay registration info for a node pub key, as returned by
+ * /api/relays (which comes back as an array of RegisteredRelayInfo).
+ * We don't wrap the individual lookup on the server — just filter the
+ * full list client-side. It's bounded (N nodes in the network) and
+ * cached heavily enough that this is cheaper than a new endpoint.
+ */
+export interface RegisteredRelayInfo {
+  pub_key:         string;
+  address:         string;
+  relay: {
+    x25519_pub_key: string;
+    fee_per_msg_ut: number;
+    multiaddr?:     string;
+  };
+  last_heartbeat?: number;  // unix seconds
+}
+
+/** GET /api/relays — all relay nodes registered on-chain. */
+export async function getRelays(): Promise<RegisteredRelayInfo[]> {
+  try {
+    return await get<RegisteredRelayInfo[]>('/api/relays');
+  } catch {
+    return [];
+  }
+}
+
+/** Find relay entry for a specific pub key. null if the address isn't a relay. */
+export async function getRelayFor(pubKey: string): Promise<RegisteredRelayInfo | null> {
+  const all = await getRelays();
+  return all.find(r => r.pub_key === pubKey) ?? null;
+}
+
 /** Fetch identity info for any pubkey or DC address. Returns null on 404. */
 export async function getIdentity(pubkeyOrAddr: string): Promise<IdentityInfo | null> {
   try {