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/ui/node_status_banner.h

@@ -0,0 +1,111 @@
+#pragma once
+
+#include <string>
+
+// Persistent node-connectivity banner shown at the top of the content column when the wallet
+// cannot reach its node. Distinct from the transient toast notifications: it stays visible for
+// as long as the fault persists, so an offline wallet is never silently mistaken for a working
+// one. The decision (whether to show, how severe, which action) is a pure function of a state
+// snapshot so it can be unit-tested; App::renderNodeStatusBanner() feeds it the live state and
+// draws the strip. See src/app.cpp.
+namespace dragonx::ui {
+
+// Visual weight. Warning (amber) = recoverable / a reconnect is offered; Error (red) = a hard
+// fault the user must act on (the daemon gave up crashing, or a lite wallet failed to open).
+enum class NodeBannerSeverity {
+    Warning,
+    Error,
+};
+
+// What the banner's action button does. App maps this to the concrete call.
+enum class NodeBannerAction {
+    None,         // no button — nothing the user can usefully do from here
+    Reconnect,    // full node: re-run the RPC connect state machine (App::tryConnect)
+    RestartNode,  // full node: the embedded daemon crashed & auto-restart gave up (App::restartDaemon)
+};
+
+// Why the banner is up. App maps this to a translated headline; `detail` carries the live,
+// already-human-readable status text (connection_status_ / daemon lastError / lite open error).
+enum class NodeBannerReason {
+    None,
+    FullNodeOffline,  // a reachable node was lost, or never came up; reconnect offered
+    DaemonCrashed,    // the embedded daemon crashed repeatedly and auto-restart stopped
+    LiteOpenFailed,   // lite build: the wallet failed to open
+};
+
+struct NodeBannerState {
+    bool               show     = false;
+    NodeBannerSeverity severity = NodeBannerSeverity::Warning;
+    NodeBannerReason   reason   = NodeBannerReason::None;
+    NodeBannerAction   action   = NodeBannerAction::None;
+    std::string        detail;  // passthrough status/error text (may be empty)
+};
+
+// Snapshot of the connection state the banner reads. Plain values so the decision is testable
+// without an App instance.
+struct NodeBannerInputs {
+    bool lite                 = false;  // lite build (no embedded daemon / RPC)
+    bool connected            = false;  // state_.connected — the master "online" flag
+    bool warming_up           = false;  // daemon reachable, RPC warmup (code -28)
+    bool daemon_initializing  = false;  // daemon launching / block index loading
+    bool connection_in_progress = false;  // a connect attempt is actively running
+
+    // Full-node embedded-daemon crash signal.
+    bool using_embedded_daemon = false;
+    bool has_daemon_controller = false;
+    bool daemon_running        = false;
+    int  daemon_crash_count    = 0;
+
+    std::string connection_status;  // human-readable status line (already translated)
+    std::string daemon_last_error;  // DaemonController::lastError() (may be empty)
+    std::string lite_open_error;    // lite: last wallet-open failure reason
+};
+
+// Auto-restart give-up threshold — mirrors the crash cap in app_network.cpp's connect loop.
+inline constexpr int kNodeBannerCrashGiveUpCount = 3;
+
+inline NodeBannerState evaluateNodeStatusBanner(const NodeBannerInputs& in) {
+    NodeBannerState s;
+
+    if (in.lite) {
+        // Lite has no daemon/RPC; "online" == wallet open. Only a genuine open failure is a
+        // fault worth a persistent banner (a not-yet-created wallet is handled by the normal
+        // "No wallet open" prompt, and leaves lite_open_error empty).
+        if (!in.connected && !in.lite_open_error.empty()) {
+            s.show     = true;
+            s.severity = NodeBannerSeverity::Error;
+            s.reason   = NodeBannerReason::LiteOpenFailed;
+            s.action   = NodeBannerAction::None;
+            s.detail   = in.lite_open_error;
+        }
+        return s;
+    }
+
+    // Full node. Connected, or in an expected startup phase → the loading/warmup overlay owns
+    // the screen, so no banner. An active connect attempt likewise shows progress, not an
+    // error — don't flicker a banner over it.
+    if (in.connected) return s;
+    if (in.warming_up || in.daemon_initializing) return s;
+    if (in.connection_in_progress) return s;
+
+    // Genuinely offline. Distinguish "the embedded daemon crashed and we stopped retrying" (a
+    // hard fault needing a manual restart) from an ordinary lost/failed connection (retryable).
+    if (in.using_embedded_daemon && in.has_daemon_controller && !in.daemon_running &&
+        in.daemon_crash_count >= kNodeBannerCrashGiveUpCount) {
+        s.show     = true;
+        s.severity = NodeBannerSeverity::Error;
+        s.reason   = NodeBannerReason::DaemonCrashed;
+        s.action   = NodeBannerAction::RestartNode;
+        s.detail   = !in.daemon_last_error.empty() ? in.daemon_last_error : in.connection_status;
+        return s;
+    }
+
+    s.show     = true;
+    s.severity = NodeBannerSeverity::Warning;
+    s.reason   = NodeBannerReason::FullNodeOffline;
+    s.action   = NodeBannerAction::Reconnect;
+    s.detail   = in.connection_status;
+    return s;
+}
+
+}  // namespace dragonx::ui