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

node/api_feed.go

@@ -34,6 +34,7 @@ import (
 	"encoding/base64"
 	"encoding/hex"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"log"
 	"net/http"
@@ -249,10 +250,16 @@ func feedPublish(cfg FeedConfig) http.HandlerFunc {
 		}
 		hashtags, err := cfg.Mailbox.Store(post, req.Ts)
 		if err != nil {
-			if err == relay.ErrPostTooLarge {
+			if errors.Is(err, relay.ErrPostTooLarge) {
 				jsonErr(w, err, 413)
 				return
 			}
+			if errors.Is(err, relay.ErrFeedQuotaExceeded) {
+				// 507 Insufficient Storage — the client should try
+				// another relay (or wait for TTL-driven eviction here).
+				jsonErr(w, err, 507)
+				return
+			}
 			jsonErr(w, err, 500)
 			return
 		}