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)/chats/index.tsx

@@ -28,6 +28,7 @@ export default function ChatsScreen() {
   const insets   = useSafeAreaInsets();
   const contacts = useStore(s => s.contacts);
   const messages = useStore(s => s.messages);
+  const keyFile  = useStore(s => s.keyFile);
 
   // Статус подключения: online / connecting / offline.
   // Название шапки и цвет pip'а на аватаре зависят от него.
@@ -48,9 +49,14 @@ export default function ChatsScreen() {
     return msgs && msgs.length ? msgs[msgs.length - 1] : null;
   };
 
-  // Сортировка по последней активности.
+  // Сортировка по последней активности. Saved Messages (self-chat) всегда
+  // закреплён сверху — это "Избранное", бессмысленно конкурировать с ним
+  // по recency'и обычным чатам.
+  const selfAddr = keyFile?.pub_key;
   const sorted = useMemo(() => {
-    return [...contacts]
+    const saved = selfAddr ? contacts.find(c => c.address === selfAddr) : undefined;
+    const rest = contacts
+      .filter(c => c.address !== selfAddr)
       .map(c => ({ c, last: lastOf(c) }))
       .sort((a, b) => {
         const ka = a.last ? a.last.timestamp : a.c.addedAt / 1000;
@@ -58,7 +64,8 @@ export default function ChatsScreen() {
         return kb - ka;
       })
       .map(x => x.c);
-  }, [contacts, messages]);
+    return saved ? [saved, ...rest] : rest;
+  }, [contacts, messages, selfAddr]);
 
   return (
     <View style={{ flex: 1, backgroundColor: '#000000', paddingTop: insets.top }}>
@@ -72,6 +79,7 @@ export default function ChatsScreen() {
             <ChatTile
               contact={item}
               lastMessage={lastOf(item)}
+              saved={item.address === selfAddr}
               onPress={() => router.push(`/(app)/chats/${item.address}` as never)}
             />
           )}