docs: relicense Sencho to AGPLv3 and reframe Community positioning (#1623)

* docs: relicense Sencho to AGPLv3 and reframe Community positioning

Replace BSL with AGPLv3 for the public Community product, update contributor
and licensing copy for Community-focused contributions, and surface source
and license links in Settings About.

* docs: clarify CLA scope and Community contribution framing

* fix(ui): split About link constants and harden AboutSection test selectors

frontend/src/components/Setup.tsx

@@ -174,7 +174,7 @@ export function Setup({ onComplete, className, ...props }: SetupProps & React.Co
           <AuthStepHeader
             kicker="SENCHO · INITIALIZE"
             hero="Cold start"
-            caption="Create the Admiral account to unlock the console."
+            caption="Create the initial administrator account for this control plane."
           />
 
           <form onSubmit={handleSubmit} className="flex flex-col gap-5">