ui: grayscale palette, square corners throughout

Removes all indigo/emerald/amber/red accent colors — everything now
uses the slate ramp. Strips rounded corners from containers, cards,
buttons, inputs, badges; the only remaining rounded element is the
circular logo image itself.

Affordances preserved without color:
- Primary buttons (Publish/Log in/Unlock/Copy link flash): bg-slate-100
  on dark background for high contrast
- Secondary buttons (Copy/Open): bg-slate-800
- Destructive (Delete): outlined with border-slate-500 so it reads as
  different from secondary
- Error messages: uppercase "ERROR" prefix + left border accent
- Success flash / ready drop state: uppercase "PUBLISHED"/"READY" labels
  and solid border-slate-200 vs. the dashed/empty state
- Protected lock: amber badge swapped for a slate-100 badge overlay
- Chart: slate-200 line on slate-400/20 fill instead of indigo

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/server/templates/detail.html

@@ -6,7 +6,7 @@
 
   <div class="flex items-start justify-between mb-6">
     <div>
-      <h1 class="text-2xl font-semibold font-mono">{{ .Artifact.Slug }} {{ if .Artifact.HasPassword }}<span class="text-base text-amber-400 align-middle">🔒</span>{{ end }}</h1>
+      <h1 class="text-2xl font-semibold font-mono">{{ .Artifact.Slug }} {{ if .Artifact.HasPassword }}<span class="text-base text-slate-400 align-middle">🔒</span>{{ end }}</h1>
       {{ if .Artifact.Title }}<p class="text-slate-400 mt-1">{{ .Artifact.Title }}</p>{{ end }}
       <p class="text-xs text-slate-500 mt-2">
         Created {{ humanTime .Artifact.CreatedAt }} ·
@@ -14,23 +14,23 @@
         {{ .Artifact.ViewCount }} views
         {{ if .Artifact.ExpiresAt }} · expires {{ humanTime .Artifact.ExpiresAt }}{{ end }}
       </p>
-      <p class="mt-2 text-sm"><a class="text-indigo-300 hover:text-indigo-200" href="{{ .BaseURL }}/p/{{ .Artifact.Slug }}" target="_blank">{{ .BaseURL }}/p/{{ .Artifact.Slug }}</a></p>
+      <p class="mt-2 text-sm"><a class="text-slate-100 underline decoration-slate-600 hover:decoration-slate-200" href="{{ .BaseURL }}/p/{{ .Artifact.Slug }}" target="_blank">{{ .BaseURL }}/p/{{ .Artifact.Slug }}</a></p>
     </div>
     <div class="flex gap-2">
       <button onclick="navigator.clipboard.writeText('{{ .BaseURL }}/p/{{ .Artifact.Slug }}')"
-              class="text-sm px-3 py-1.5 rounded bg-slate-800 hover:bg-slate-700">Copy link</button>
+              class="text-sm px-3 py-1.5 bg-slate-800 hover:bg-slate-700">Copy link</button>
       <button hx-delete="/a/{{ .Artifact.Slug }}" hx-confirm="Delete {{ .Artifact.Slug }}? This cannot be undone."
               hx-on::after-request="window.location='/'"
-              class="text-sm px-3 py-1.5 rounded bg-red-900/60 hover:bg-red-800">Delete</button>
+              class="text-sm px-3 py-1.5 border border-slate-500 text-slate-200 hover:bg-slate-800">Delete</button>
     </div>
   </div>
 
-  <section class="rounded border border-slate-800 bg-slate-900/30 p-4 mb-6">
+  <section class="border border-slate-800 bg-slate-900/30 p-4 mb-6">
     <h2 class="text-sm uppercase tracking-wide text-slate-400 mb-3">Views — last 30 days</h2>
     {{ .ChartSVG }}
   </section>
 
-  <section class="rounded border border-slate-800 bg-slate-900/30 p-4">
+  <section class="border border-slate-800 bg-slate-900/30 p-4">
     <h2 class="text-sm uppercase tracking-wide text-slate-400 mb-3">Top referrers</h2>
     {{ if .Referrers }}
     <table class="w-full text-sm">