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

web/templates/playlist.html

@@ -4,12 +4,15 @@
     <h2 class="h5 mb-0">{{.Data.Playlist.Name}}</h2>
     {{if .Data.Playlist.Description}}<div class="text-secondary small">{{.Data.Playlist.Description}}</div>{{end}}
   </div>
-  {{if .Data.Owned}}
-  <form method="post" action="/playlists/{{.Data.Playlist.ID}}/delete" onsubmit="return confirm('Delete this playlist?');">
-    {{template "csrf_field" .CSRFToken}}
-    <button type="submit" class="btn btn-outline-danger btn-sm"><i class="bi bi-trash"></i> Delete</button>
-  </form>
-  {{end}}
+  <div class="d-flex gap-2">
+    <a href="/playlists/{{.Data.Playlist.ID}}/export" class="btn btn-outline-secondary btn-sm"><i class="bi bi-download"></i> Export M3U</a>
+    {{if .Data.Owned}}
+    <form method="post" action="/playlists/{{.Data.Playlist.ID}}/delete" onsubmit="return confirm('Delete this playlist?');">
+      {{template "csrf_field" .CSRFToken}}
+      <button type="submit" class="btn btn-outline-danger btn-sm"><i class="bi bi-trash"></i> Delete</button>
+    </form>
+    {{end}}
+  </div>
 </div>
 {{if .Data.Songs}}{{template "song_rows" .Data.Songs}}{{else}}<p class="text-secondary">This playlist is empty.</p>{{end}}
 {{template "footer" .}}{{end}}