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/scripts/build.js

@@ -1,5 +1,5 @@
 /**
- * build.js — Build script for the Nexus Terminal frontend.
+ * build.js — Build script for the Alchementary Link frontend.
  *
  * In production mode (no flags): copies vendor assets, assembles the
  * dist/ directory with all files needed by the Wails embed, and validates