Rebrand: Nexus Terminal → Alchementary Link

Renames the product across the codebase: Go module path, config/data
paths, keychain service name, export file extension, build tooling,
docs, and UI text. Sidebar brand mark now reads ">_ AlchementaryLink"
with "Alchementary" in the connection-group gold accent colour, and
the window title is "Alchementary Link".

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019T8bJS8BCtY5GeSG2HBj1n

frontend/index.html

@@ -3,7 +3,7 @@
   <head>
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
-    <title>Nexus Terminal</title>
+    <title>Alchementary Link</title>
 
     <!-- Vendor styles -->
     <link rel="stylesheet" href="/src/css/lib/xterm.css" />
@@ -52,10 +52,10 @@
             <span class="sidebar-product-switch" aria-label="Application mode">
               <button
                 class="sidebar-title sidebar-mode-toggle is-active"
-                id="mode-nexus"
+                id="mode-link"
                 type="button"
               >
-                Nexus Terminal
+                <span class="sidebar-brand-gold">Alchementary</span>Link
               </button>
               <button
                 class="sidebar-title sidebar-mode-toggle"