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/unlock.html

@@ -5,8 +5,8 @@
     <div class="flex flex-col items-center text-center mb-8">
       <div class="relative mb-4">
         <img src="/static/logo.png" alt=""
-             class="w-20 h-20 rounded-full ring-1 ring-slate-700 shadow-lg shadow-indigo-500/10">
-        <span class="absolute -bottom-1 -right-1 inline-flex items-center justify-center w-8 h-8 rounded-full bg-amber-500 text-slate-950 ring-2 ring-slate-950">
+             class="w-20 h-20 rounded-full ring-1 ring-slate-700 shadow-lg shadow-black/40">
+        <span class="absolute -bottom-1 -right-1 inline-flex items-center justify-center w-8 h-8 bg-slate-100 text-slate-950 ring-2 ring-slate-950">
           <svg class="w-4 h-4" fill="none" stroke="currentColor" stroke-width="2.5" viewBox="0 0 24 24">
             <path stroke-linecap="round" stroke-linejoin="round"
                   d="M12 11v2m-4 4h8a2 2 0 002-2v-5a2 2 0 00-2-2H8a2 2 0 00-2 2v5a2 2 0 002 2zM10 9V7a2 2 0 114 0v2"/>
@@ -18,16 +18,18 @@
     </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="/p/{{ .Slug }}/unlock" class="space-y-3">
       <label class="block">
         <span class="block text-sm text-slate-400 mb-1">Password</span>
         <input type="password" name="password" required autofocus autocomplete="off"
-               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">Unlock</button>
+      <button class="w-full bg-slate-100 hover:bg-white text-slate-950 px-3 py-2 font-medium">Unlock</button>
     </form>
   </div>
 </main>