feat(diagnostics): persistent node/RPC error banner at top of content (Foundation QoL)

A persistent horizontal strip now appears at the top of the content column whenever the
wallet can't reach its node — unlike the transient toasts it stays up for as long as the
fault persists, so an offline wallet is never silently mistaken for a working one.

The show/severity/action decision is a pure, unit-tested function
(ui/node_status_banner.h::evaluateNodeStatusBanner) fed a state snapshot by the new
App::renderNodeStatusBanner(). Three cases:
  - full-node offline        -> amber, "Reconnect"    (App::tryConnect)
  - embedded daemon crashed
    & auto-restart gave up    -> red,   "Restart node" (App::restartDaemon)
  - lite wallet open failed   -> red,   message-only

Suppressed during the wizard / wallet-switch / daemon-restart / screenshot-sweep / shutdown,
and while an expected startup phase (warmup / init / connect-in-progress) already owns the
screen. Banner height lives in res/themes/ui.toml (banners.node-status); colours come from the
material semantic palette; the detail text is ellipsis-clipped so it can't push the action
button off-screen. Drawn before the content edge-fade vertex capture so it stays fully opaque.

New i18n keys (node_banner_*). Build-clean both variants; ctest 1/1 (adds testNodeStatusBanner).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/util/i18n.cpp

@@ -1309,6 +1309,12 @@ void I18n::loadBuiltinEnglish()
         strings_["sb_connecting_err"] = "Connecting to daemon — %s";
         strings_["sb_daemon_crashed"] = "Daemon crashed %d times";
         strings_["sb_daemon_start_failed"] = "Couldn't start dragonxd";
+        // Persistent node-status banner (App::renderNodeStatusBanner).
+        strings_["node_banner_offline_title"] = "Not connected to the DragonX node";
+        strings_["node_banner_crashed_title"] = "The node stopped unexpectedly";
+        strings_["node_banner_lite_open_failed"] = "Couldn't open your wallet";
+        strings_["node_banner_reconnect"] = "Reconnect";
+        strings_["node_banner_restart"] = "Restart node";
         strings_["daemon_port_busy_warn"] =
             "Port " DRAGONX_DEFAULT_RPC_PORT " is in use but isn't responding as a DragonX node. "
             "Close the program using it (or free the port), then restart — the wallet can't start "