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/package.json

@@ -1,8 +1,8 @@
 {
-  "name": "nexus-terminal-frontend",
+  "name": "alchementary-link-frontend",
   "version": "0.1.0",
   "private": true,
-  "description": "Frontend for Nexus Terminal desktop SSH client",
+  "description": "Frontend for Alchementary Link desktop SSH client",
   "scripts": {
     "install:assets": "node scripts/copy-assets.js",
     "bundle:novnc": "node scripts/bundle-novnc.js",