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/models/defaults.go

@@ -128,8 +128,8 @@ func NewProfile(protocol Protocol) Profile {
 	case ProtocolIRC:
 		p.Port = DefaultIRCPort
 		p.IRC = &IRCConfig{
-			Nick:               "nexus",
-			RealName:           "Nexus Terminal",
+			Nick:               "link",
+			RealName:           "Alchementary Link",
 			ServerPasswordMode: SecretModeAskEveryTime,
 			NickServMode:       SecretModeSaved,
 			AutoJoinRooms:      []string{},

internal/models/models.go

@@ -1,5 +1,5 @@
 // Package models defines the core data structures used throughout
-// the Nexus Terminal application.
+// the Alchementary Link application.
 package models
 
 import "time"