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

internal/storage/migrations/001_initial.sql

@@ -1,4 +1,4 @@
--- Initial schema for Nexus Terminal
+-- Initial schema for Alchementary Link
 -- Creates the core tables for profiles, known hosts, preferences, and audit log.
 
 CREATE TABLE IF NOT EXISTS profiles (