feat(banner): per-campaign colour overrides and urgent alerts

Non-default campaigns gain an optional colour override (background / text /
link) in two sets — one for every theme, one for the dark theme only — and an
"urgent" flag.

- Colours ride profile.get as six trailing FlatBuffers strings on
  BannerCampaign (backward-compatible). The client resolves the cascade
  (dark <- dark ?? all, light <- all) per rendered theme and derives the strip
  border from the background in JS (no CSS color-mix, for the old Android
  WebView floor); AdBanner applies them as inline vars scoped to the strip.
- Urgent is resolved entirely server-side: while any enabled, in-window urgent
  campaign exists, computeActiveSet returns only the urgent campaigns and
  bannerFor skips the eligibility gate — so a system notice reaches every viewer
  (paid / hint-holding / no_banner included) and preempts the ordinary feed. No
  wire field; it appears on each viewer's next profile.get.
- Admin console (/_gm/banners): native colour pickers + a live light/dark
  preview of the strip, and an urgent toggle. The default campaign stays plain,
  enforced by the service and a DB CHECK.

Migration 00009 is additive (nullable colour columns + a bool default +
all-or-nothing / hex / default-plain CHECKs) — expand-contract, rollback-safe.

Docs: ARCHITECTURE §10, UI_DESIGN, FUNCTIONAL (+ru). Tests: ads unit (urgent
preempt + colour validation), codec + resolver unit, gateway transcode, and
integration (colour round-trip + urgent bypass against real Postgres).

ui/src/lib/bannerEngine.ts

@@ -4,11 +4,14 @@
 // independent of route transitions (the cycle is not restarted on navigation).
 
 import { createBannerRotator, type BannerHost, type Rotator } from './banner';
-import type { BannerCampaign, BannerTimings } from './model';
+import type { BannerCampaign, BannerColors, BannerTimings } from './model';
+
+const noColors: BannerColors = { all: null, dark: null };
 
 let rotator: Rotator | null = null;
 let mounted: BannerHost | null = null;
 let current = '';
+let currentColors: BannerColors = noColors;
 let key = '';
 // The in-flight horizontal scroll of the current message, so a view mounted by navigation can
 // resume it from the same offset instead of restarting at the left. Null when not scrolling.
@@ -17,10 +20,11 @@ let activeScroll: { toPx: number; dur: number; start: number } | null = null;
 // proxy is the rotator's host: it records the current message + scroll (so a freshly-mounted view
 // can resume them) and forwards every effect to the currently-attached DOM host, if any.
 const proxy: BannerHost = {
-  show(md) {
+  show(md, colors) {
     current = md;
+    currentColors = colors;
     activeScroll = null;
-    mounted?.show(md);
+    mounted?.show(md, colors);
   },
   resetScroll() {
     activeScroll = null;
@@ -58,6 +62,7 @@ export function configureBanner(campaigns: BannerCampaign[], timings: BannerTimi
   key = k;
   rotator?.stop();
   current = '';
+  currentColors = noColors;
   rotator = createBannerRotator(campaigns, proxy, timings);
   rotator.start();
 }
@@ -71,6 +76,15 @@ export function bannerCurrent(): string {
   return current;
 }
 
+/**
+ * bannerCurrentColors returns the colour override of the campaign whose message is currently
+ * displayed (neutral tokens — both sets null — before the first message). A freshly-mounted view
+ * reads it alongside bannerCurrent so the strip paints the right colours immediately on mount.
+ */
+export function bannerCurrentColors(): BannerColors {
+  return currentColors;
+}
+
 /**
  * attachBannerHost connects a freshly-mounted view as the DOM host. It does NOT re-show the
  * current message (the view renders it itself from bannerCurrent on mount): re-showing here would