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/model.ts

@@ -169,12 +169,33 @@ export interface Banner {
   timings: BannerTimings;
 }
 
+/** ColorTriple is a banner colour override's three "#rrggbb" colours. */
+export interface ColorTriple {
+  bg: string;
+  fg: string;
+  link: string;
+}
+
+/**
+ * BannerColors is a campaign's optional palette: `all` overrides every theme, `dark`
+ * further overrides the dark theme (either may be null — the neutral tokens then apply).
+ * resolveBannerColors (lib/bannerColors) collapses it to the rendered theme's colours.
+ */
+export interface BannerColors {
+  all: ColorTriple | null;
+  dark: ColorTriple | null;
+}
+
 /** BannerCampaign is one campaign in the rotation feed. */
 export interface BannerCampaign {
   /** GCD-reduced show weight for the client's smooth weighted round-robin. */
   weight: number;
   /** Messages in display order, already resolved to the viewer's bot language. */
   messages: string[];
+  /** Colour override for every theme; null (or absent) keeps the neutral tokens. */
+  overrideAll?: ColorTriple | null;
+  /** Colour override for the dark theme only; null (or absent) falls back to overrideAll/tokens. */
+  overrideDark?: ColorTriple | null;
 }
 
 /** BannerTimings are the global banner display timings (ms, except scrollPxPerSec). */