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>

res/themes/ui.toml

@@ -700,6 +700,12 @@ status-pill-bg-alpha = { size = 30 }
 status-pill-y-offset = { size = 1 }
 confirmed-threshold = { size = 10 }
 
+# Persistent node/RPC error strip at the top of the content column (see App::renderNodeStatusBanner).
+# Slightly taller than the per-tab sync banner so it comfortably holds the Reconnect/Restart action.
+[banners.node-status]
+min-height = { size = 26.0 }
+height = { size = 30.0 }
+
 [tabs.transactions]
 search-max-width = 300.0
 search-width-ratio = 0.3