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/banner.ts

@@ -4,7 +4,7 @@
 // unit-testable with fake timers. The campaigns + timings come from the profile's
 // banner block (see lib/model, ARCHITECTURE §10); the host is AdBanner.svelte.
 
-import type { BannerCampaign, BannerTimings } from './model';
+import type { BannerCampaign, BannerColors, BannerTimings } from './model';
 
 /** Fallback display timings, matching the seeded ad_settings defaults. */
 export const defaultBannerTimings: BannerTimings = {
@@ -16,9 +16,15 @@ export const defaultBannerTimings: BannerTimings = {
   fadeInMs: 1_000,
 };
 
+/** BannerItem is one scheduled message plus its campaign's colour override (for the strip). */
+export interface BannerItem {
+  md: string;
+  colors: BannerColors;
+}
+
 /** Scheduler yields the next message to show, fairly across weighted campaigns. */
 export interface Scheduler {
-  next(): string;
+  next(): BannerItem;
   /** Total messages across all (non-empty, positive-weight) campaigns. */
   readonly total: number;
 }
@@ -33,15 +39,17 @@ export interface Scheduler {
  */
 export function createScheduler(campaigns: BannerCampaign[]): Scheduler {
   const cs = campaigns.filter((c) => c.weight > 0 && c.messages.length > 0);
+  const colors: BannerColors[] = cs.map((c) => ({ all: c.overrideAll ?? null, dark: c.overrideDark ?? null }));
   const current = cs.map(() => 0); // SWRR running weights
   const cursor = cs.map(() => 0); // per-campaign round-robin position
   const totalWeight = cs.reduce((sum, c) => sum + c.weight, 0);
   const total = cs.reduce((sum, c) => sum + c.messages.length, 0);
+  const empty: BannerColors = { all: null, dark: null };
 
   return {
     total,
-    next(): string {
-      if (cs.length === 0) return '';
+    next(): BannerItem {
+      if (cs.length === 0) return { md: '', colors: empty };
       let best = 0;
       for (let i = 0; i < cs.length; i++) {
         current[i] += cs[i].weight;
@@ -51,15 +59,16 @@ export function createScheduler(campaigns: BannerCampaign[]): Scheduler {
       const c = cs[best];
       const md = c.messages[cursor[best] % c.messages.length];
       cursor[best]++;
-      return md;
+      return { md, colors: colors[best] };
     },
   };
 }
 
 /** The host the rotator drives; AdBanner.svelte supplies the DOM measurements and effects. */
 export interface BannerHost {
-  /** Render md and fade it in (over fadeInMs), with the scroll reset to the start. */
-  show(md: string): void;
+  /** Render md and fade it in (over fadeInMs), with the scroll reset to the start. colors is the
+   *  source campaign's colour override, applied to the strip (its neutral tokens when unset). */
+  show(md: string, colors: BannerColors): void;
   /** Reset the scroll to the start without re-fading (to loop a long message). */
   resetScroll(): void;
   /** Fade the current message out over durationMs. */
@@ -98,7 +107,7 @@ export function createBannerRotator(
   const sched = createScheduler(campaigns);
   let running = false;
   let cycleStart = 0;
-  let lastShown = ''; // the message currently presented, for restart() (re-measure)
+  let lastShown: BannerItem | null = null; // the item currently presented, for restart() (re-measure)
   const timers: ReturnType<typeof setTimeout>[] = [];
 
   const at = (ms: number, fn: () => void) => {
@@ -109,11 +118,11 @@ export function createBannerRotator(
     timers.length = 0;
   };
 
-  function present(md: string) {
+  function present(item: BannerItem) {
     if (!running) return;
     clear();
-    lastShown = md;
-    host.show(md);
+    lastShown = item;
+    host.show(item.md, item.colors);
     at(config.fadeInMs, measure);
   }
 
@@ -143,7 +152,7 @@ export function createBannerRotator(
           // the same fade as a message change, not a hard jump.
           host.hide(config.fadeOutMs);
           at(config.fadeOutMs + config.gapMs, () => {
-            host.show(lastShown);
+            if (lastShown) host.show(lastShown.md, lastShown.colors);
             at(config.fadeInMs, () => scrollCycle(over));
           });
         }