Complete Phase 4: security hardening, player/playlist/admin UI, docs

Security:
- New internal/netguard package: podcast feed/enclosure fetching now
  refuses to dial non-public IPs (loopback/RFC1918/link-local/cloud
  metadata/CGNAT), validated per-connection including across redirects,
  closing an SSRF gap where a malicious or compromised feed URL could
  probe the server's internal network. Configurable via
  VIBESONIC_PODCASTS_ALLOW_PRIVATE_NETWORKS for intentional LAN feeds.
- Podcast episode downloads capped at 4GB (was unbounded, disk-exhaustion risk)
- Cover art decoding now rejects images above a 40-megapixel budget
  before full decode (decompression-bomb guard)
- HTTP server sets ReadHeaderTimeout (slowloris hardening)
- Admin folder-scan redirect now validated as same-site instead of
  trusting the raw Referer header (open-redirect fix)
- Legacy Subsonic token/salt auth now uses a constant-time comparison

Web UI completion:
- Persistent player: shuffle and repeat (off/all/one) controls, wired
  into queue advance/prev logic and persisted like other player state
- Admin user-permissions form: adminRole toggle (with a guard against
  demoting the last remaining admin) and per-user music-folder access
  checkboxes, backed by existing schema/enforcement that was previously
  unreachable from the UI
- Playlist M3U8 import/export

Docs: added docs/client-compatibility.md (honest tracking template —
no client testing has been performed), docs/deployment.md,
docs/security.md, docs/backup.md; refreshed README and cross-links;
added a footnote in docs/subsonic-compatibility.md distinguishing
spec-standard from VibeSonic-original OpenSubsonic extensions.

Also added Go fuzz tests for the external-auth CIDR/IP parser and the
LRC lyrics parser.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014r98AtcBbevpscWLsfwVvv

internal/netguard/netguard_fuzz_test.go

@@ -0,0 +1,53 @@
+package netguard
+
+import (
+	"net"
+	"testing"
+)
+
+// FuzzValidateURL guards against a crash in the podcast feed/enclosure
+// URL sanity check — the URLs it validates are admin-supplied but the
+// check must never panic regardless of input.
+func FuzzValidateURL(f *testing.F) {
+	f.Add("https://example.com/feed.xml")
+	f.Add("http://127.0.0.1/feed")
+	f.Add("ftp://example.com")
+	f.Add("not a url at all")
+	f.Add("")
+	f.Add("http://")
+	f.Add("javascript:alert(1)")
+	f.Add("http://[::1]:8080/feed")
+
+	f.Fuzz(func(t *testing.T, raw string) {
+		defer func() {
+			if r := recover(); r != nil {
+				t.Fatalf("ValidateURL panicked on %q: %v", raw, r)
+			}
+		}()
+		_ = ValidateURL(raw)
+	})
+}
+
+// FuzzIsPublicIP guards the address classifier that decides whether a
+// resolved IP is safe to connect to.
+func FuzzIsPublicIP(f *testing.F) {
+	f.Add("8.8.8.8")
+	f.Add("127.0.0.1")
+	f.Add("::1")
+	f.Add("169.254.169.254")
+	f.Add("100.64.0.1")
+	f.Add("::ffff:127.0.0.1")
+
+	f.Fuzz(func(t *testing.T, s string) {
+		ip := net.ParseIP(s)
+		if ip == nil {
+			return
+		}
+		defer func() {
+			if r := recover(); r != nil {
+				t.Fatalf("IsPublicIP panicked on %q: %v", s, r)
+			}
+		}()
+		IsPublicIP(ip)
+	})
+}