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/components/Avatar.tsx

@@ -6,6 +6,7 @@
  */
 import React from 'react';
 import { View, Text } from 'react-native';
+import { Ionicons } from '@expo/vector-icons';
 
 export interface AvatarProps {
   /** Имя / @username — берём первый символ для placeholder. */
@@ -18,6 +19,11 @@ export interface AvatarProps {
   dotColor?: string;
   /** Класс для обёртки (position: relative кадр). */
   className?: string;
+  /**
+   * Saved Messages variant — blue circle with a bookmark glyph, Telegram-style.
+   * When set, `name`/`address` are ignored for the visual.
+   */
+  saved?: boolean;
 }
 
 /** Простое хэширование имени → один из 6 оттенков серого для разнообразия. */
@@ -28,10 +34,10 @@ function pickBg(seed: string): string {
   return shades[h % shades.length];
 }
 
-export function Avatar({ name, address, size = 48, dotColor, className }: AvatarProps) {
+export function Avatar({ name, address, size = 48, dotColor, className, saved }: AvatarProps) {
   const seed = (name ?? address ?? '?').replace(/^@/, '');
   const initial = seed.charAt(0).toUpperCase() || '?';
-  const bg = pickBg(seed);
+  const bg = saved ? '#1d9bf0' : pickBg(seed);
 
   return (
     <View className={className} style={{ width: size, height: size, position: 'relative' }}>
@@ -45,16 +51,20 @@ export function Avatar({ name, address, size = 48, dotColor, className }: Avatar
           justifyContent: 'center',
         }}
       >
-        <Text
-          style={{
-            color: '#d0d0d0',
-            fontSize: size * 0.4,
-            fontWeight: '600',
-            includeFontPadding: false,
-          }}
-        >
-          {initial}
-        </Text>
+        {saved ? (
+          <Ionicons name="bookmark" size={size * 0.5} color="#ffffff" />
+        ) : (
+          <Text
+            style={{
+              color: '#d0d0d0',
+              fontSize: size * 0.4,
+              fontWeight: '600',
+              includeFontPadding: false,
+            }}
+          >
+            {initial}
+          </Text>
+        )}
       </View>
       {dotColor && (
         <View