feat(apple/library): experimental game-library browser (flagged off)

Plan step 3 — the Apple client surfaces the host's game library, behind a feature
flag (`DefaultsKey.libraryEnabled`, default OFF). Browsing only; launching a chosen
title is step 4.

- PunktfunkKit `LibraryClient`: Codable GameEntry/Artwork/LaunchSpec mirroring
  crates/punktfunk-host/src/library.rs, and an async fetch of GET /api/v1/library
  with a bearer token. Typed LibraryError guides setup (the common case is "needs a
  --mgmt-token"). `Artwork.posterCandidates` = portrait → header → hero.
- `LibraryView`: cross-platform poster grid (LazyVGrid, AsyncImage that walks the art
  candidates past load failures to a text placeholder), a store badge, and an inline
  Connection form (mgmt port + token) that surfaces when the API is unreachable / 401
  / no token set. Read-only.
- StoredHost gains `mgmtPort`/`mgmtToken` (the mgmt API is a distinct port from the
  data plane and needs a token off-loopback). Both OPTIONAL — synthesized Decodable
  ignores property defaults but treats a missing Optional as nil, so older saved
  hosts decode unchanged (a defaulted non-optional would wipe the list). HostStore.setMgmt.
- Entry point: a flag-gated "Browse Library…" host-card context action → LibraryView
  (sheet on macOS/iOS, pushed on tvOS), mirroring the pair/speed-test plumbing. Plus a
  Settings "Experimental" toggle.

Can't compile Swift on the Linux dev box; CI (apple.yml: swift build + swift test on
the mac mini) verifies the macOS path. Added LibraryClientTests (decode + art order)
for `swift test`. iOS/tvOS-only branches mirror existing patterns. Live-verify on the
Mac pending.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

clients/apple/Sources/PunktfunkClient/SettingsView.swift

@@ -19,6 +19,7 @@ struct SettingsView: View {
     @AppStorage(DefaultsKey.bitrateKbps) private var bitrateKbps = 0
     @AppStorage(DefaultsKey.presenter) private var presenter = "stage1"
     @AppStorage(DefaultsKey.cursorMode) private var cursorMode = "auto"
+    @AppStorage(DefaultsKey.libraryEnabled) private var libraryEnabled = false
     @AppStorage(DefaultsKey.micEnabled) private var micEnabled = true
     @ObservedObject private var gamepads = GamepadManager.shared
     #if os(macOS)
@@ -405,6 +406,18 @@ struct SettingsView: View {
                     .font(.caption)
                     .foregroundStyle(.secondary)
             }
+            Section {
+                Toggle("Show game library", isOn: $libraryEnabled)
+            } header: {
+                Text("Experimental")
+            } footer: {
+                Text("Adds a “Browse Library…” action to each host that lists its games "
+                    + "(Steam + custom) via the host's management API. The host must expose that "
+                    + "API on the LAN with a token (serve --mgmt-bind 0.0.0.0 --mgmt-token …). "
+                    + "Browsing only for now — launching a title comes later.")
+                    .font(.caption)
+                    .foregroundStyle(.secondary)
+            }
             Section {
                 if gamepads.controllers.isEmpty {
                     Text("No controllers detected")