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).

backend/internal/adminconsole/templates/pages/banner_detail.gohtml

@@ -11,10 +11,72 @@
 <label>Starts (UTC) <input type="datetime-local" name="starts_at" value="{{.StartsAt}}"></label>
 <label>Ends (UTC) <input type="datetime-local" name="ends_at" value="{{.EndsAt}}"></label>
 <label><input type="checkbox" name="enabled"{{if .Enabled}} checked{{end}}> Enabled</label>
+<fieldset class="ovr">
+<legend><label><input type="checkbox" name="urgent"{{if .Urgent}} checked{{end}}> Urgent</label></legend>
+<p class="note">Shows to <em>everyone</em>, always — bypassing paid accounts, hint wallets and the no-banner role. While any urgent campaign is live it is the only thing the strip shows (other campaigns and the default are suppressed). For system alerts.</p>
+</fieldset>
+<fieldset class="ovr" data-ovr-group>
+<legend><label><input type="checkbox" name="override_all_on" data-ovr="all"{{if .OverrideAllOn}} checked{{end}}> Colour override — all themes</label></legend>
+<div class="swatches">
+<label>Background <input type="color" name="override_bg" value="{{.AllBg}}" data-ovr-color{{if not .OverrideAllOn}} disabled{{end}}><span class="hex"></span></label>
+<label>Text <input type="color" name="override_fg" value="{{.AllFg}}" data-ovr-color{{if not .OverrideAllOn}} disabled{{end}}><span class="hex"></span></label>
+<label>Link <input type="color" name="override_link" value="{{.AllLink}}" data-ovr-color{{if not .OverrideAllOn}} disabled{{end}}><span class="hex"></span></label>
+</div>
+</fieldset>
+<fieldset class="ovr" data-ovr-group>
+<legend><label><input type="checkbox" name="override_dark_on" data-ovr="dark"{{if .OverrideDarkOn}} checked{{end}}> Colour override — dark theme only</label></legend>
+<div class="swatches">
+<label>Background <input type="color" name="override_bg_dark" value="{{.DarkBg}}" data-ovr-color{{if not .OverrideDarkOn}} disabled{{end}}><span class="hex"></span></label>
+<label>Text <input type="color" name="override_fg_dark" value="{{.DarkFg}}" data-ovr-color{{if not .OverrideDarkOn}} disabled{{end}}><span class="hex"></span></label>
+<label>Link <input type="color" name="override_link_dark" value="{{.DarkLink}}" data-ovr-color{{if not .OverrideDarkOn}} disabled{{end}}><span class="hex"></span></label>
+</div>
+</fieldset>
 {{end}}
 <div><button type="submit">Save</button></div>
 </form>
 {{if not .IsDefault}}
+<div class="banner-preview">
+<div class="bp"><span class="bp-label">Light theme</span><div class="ad-frame light"><div class="ad-sample" id="prev-light"><span>Sample banner text — <span class="ad-link">a link</span></span></div></div></div>
+<div class="bp"><span class="bp-label">Dark theme</span><div class="ad-frame dark"><div class="ad-sample" id="prev-dark"><span>Sample banner text — <span class="ad-link">a link</span></span></div></div></div>
+</div>
+<p class="note">Live preview of the strip on both themes. An empty override falls back to the neutral theme colours; the top/bottom border is derived from the background.</p>
+<script>
+(function(){
+  // Neutral fallbacks — mirror ui/src/app.css (--ad-bg / --text-muted / --accent).
+  var TOK={light:{bg:'#e3e7ee',fg:'#6b7280',link:'#2f6df6'},dark:{bg:'#272f3c',fg:'#9aa3b2',link:'#5b8cff'}};
+  function byName(n){return document.querySelector('[name="'+n+'"]');}
+  var allOn=byName('override_all_on'), darkOn=byName('override_dark_on');
+  if(!allOn||!darkOn){return;}
+  var f={ab:byName('override_bg'),af:byName('override_fg'),al:byName('override_link'),db:byName('override_bg_dark'),df:byName('override_fg_dark'),dl:byName('override_link_dark')};
+  var lightEl=document.getElementById('prev-light'), darkEl=document.getElementById('prev-dark');
+  function hexToRgb(h){h=h.replace('#','');return [parseInt(h.slice(0,2),16),parseInt(h.slice(2,4),16),parseInt(h.slice(4,6),16)];}
+  function pad(x){x=Math.max(0,Math.min(255,Math.round(x))).toString(16);return x.length<2?'0'+x:x;}
+  function rgbToHex(r){return '#'+pad(r[0])+pad(r[1])+pad(r[2]);}
+  function mix(a,b,t){return [a[0]+(b[0]-a[0])*t,a[1]+(b[1]-a[1])*t,a[2]+(b[2]-a[2])*t];}
+  function lum(r){return (0.2126*r[0]+0.7152*r[1]+0.0722*r[2])/255;}
+  // Derived border: nudge the background 14% toward black on a light bg, toward white on a dark bg.
+  function border(bg){var r=hexToRgb(bg);return rgbToHex(mix(r, lum(r)>0.5?[0,0,0]:[255,255,255], 0.14));}
+  function paint(el,c){
+    el.style.background=c.bg; el.style.color=c.fg;
+    el.style.borderTop='1px solid '+border(c.bg); el.style.borderBottom='1px solid '+border(c.bg);
+    var a=el.querySelector('.ad-link'); if(a){a.style.color=c.link;}
+  }
+  function resolve(){
+    var light=allOn.checked?{bg:f.ab.value,fg:f.af.value,link:f.al.value}:TOK.light;
+    var dark=darkOn.checked?{bg:f.db.value,fg:f.df.value,link:f.dl.value}
+            :(allOn.checked?{bg:f.ab.value,fg:f.af.value,link:f.al.value}:TOK.dark);
+    paint(lightEl,light); paint(darkEl,dark);
+    document.querySelectorAll('.ovr .swatches label').forEach(function(lab){
+      var inp=lab.querySelector('input[type=color]'), hx=lab.querySelector('.hex');
+      if(inp&&hx){hx.textContent=inp.value;}
+    });
+  }
+  function toggleGroup(chk){chk.closest('fieldset').querySelectorAll('[data-ovr-color]').forEach(function(inp){inp.disabled=!chk.checked;});}
+  [allOn,darkOn].forEach(function(chk){chk.addEventListener('change',function(){toggleGroup(chk);resolve();});});
+  Object.keys(f).forEach(function(k){f[k].addEventListener('input',resolve);});
+  resolve();
+})();
+</script>
 <form class="form" method="post" action="/_gm/banners/{{.ID}}/delete" onsubmit="return confirm('Delete this campaign and its messages?')">
 <button type="submit" class="danger">Delete campaign</button>
 </form>