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/login.html

@@ -4,13 +4,15 @@
   <div class="w-full max-w-sm">
     <div class="flex flex-col items-center text-center mb-8">
       <img src="/static/logo.png" alt=""
-           class="w-20 h-20 rounded-full ring-1 ring-slate-700 mb-4 shadow-lg shadow-indigo-500/10">
+           class="w-20 h-20 rounded-full ring-1 ring-slate-700 mb-4 shadow-lg shadow-black/40">
       <h1 class="text-3xl font-semibold tracking-tight">Artifacto</h1>
       <p class="text-sm text-slate-400 mt-1">self-hosted HTML artifact publisher</p>
     </div>
 
     {{ if .Error }}
-      <div class="rounded bg-red-900/40 border border-red-800 text-red-200 px-3 py-2 mb-3 text-sm">{{ .Error }}</div>
+      <div class="border-l-2 border-slate-100 bg-slate-900 text-slate-100 px-3 py-2 mb-3 text-sm">
+        <span class="uppercase tracking-wider text-xs font-bold text-slate-300 mr-2">Error</span>{{ .Error }}
+      </div>
     {{ end }}
 
     <form method="post" action="/login" class="space-y-3">
@@ -18,9 +20,9 @@
       <label class="block">
         <span class="block text-sm text-slate-400 mb-1">Password</span>
         <input type="password" name="password" required autofocus autocomplete="current-password"
-               class="w-full bg-slate-900 border border-slate-800 rounded px-3 py-2 focus:outline-none focus:border-indigo-500">
+               class="w-full bg-slate-900 border border-slate-800 px-3 py-2 focus:outline-none focus:border-slate-400">
       </label>
-      <button class="w-full bg-indigo-600 hover:bg-indigo-500 text-white rounded px-3 py-2 font-medium">Log in</button>
+      <button class="w-full bg-slate-100 hover:bg-white text-slate-950 px-3 py-2 font-medium">Log in</button>
     </form>
   </div>
 </main>