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/app/(app)/_layout.tsx

@@ -25,6 +25,7 @@ import { useGlobalInbox } from '@/hooks/useGlobalInbox';
 import { getWSClient } from '@/lib/ws';
 import { NavBar } from '@/components/NavBar';
 import { AnimatedSlot } from '@/components/AnimatedSlot';
+import { saveContact } from '@/lib/storage';
 
 export default function AppLayout() {
   const keyFile  = useStore(s => s.keyFile);
@@ -49,6 +50,23 @@ export default function AppLayout() {
   useNotifications();   // permission + tap-handler
   useGlobalInbox();     // global inbox listener → notifications on new peer msg
 
+  // Ensure the Saved Messages (self-chat) contact exists as soon as the user
+  // is signed in, so it shows up in the chat list without any prior action.
+  const contacts      = useStore(s => s.contacts);
+  const upsertContact = useStore(s => s.upsertContact);
+  useEffect(() => {
+    if (!keyFile) return;
+    if (contacts.some(c => c.address === keyFile.pub_key)) return;
+    const saved = {
+      address:   keyFile.pub_key,
+      x25519Pub: keyFile.x25519_pub,
+      alias:     'Saved Messages',
+      addedAt:   Date.now(),
+    };
+    upsertContact(saved);
+    saveContact(saved).catch(() => { /* best-effort — re-added next boot anyway */ });
+  }, [keyFile, contacts, upsertContact]);
+
   useEffect(() => {
     const ws = getWSClient();
     if (keyFile) ws.setAuthCreds({ pubKey: keyFile.pub_key, privKey: keyFile.priv_key });