refactor(brand): rename F∆I -> Ch∆In + hub binary fai -> chain

Studio follows the platform rename: product branding F∆I -> Ch∆In in UI
strings, command examples fai -> chain, and — critically — the spawned
hub binary path ~/.fai/bin/fai -> ~/.fai/bin/chain so Studio launches
the renamed binary. The fai_* Dart identifiers (FaiLog, widget files,
the generated SDK) stay = vendor/internal namespace. flutter analyze:
no issues.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

assets/docs/architecture.md

@@ -1,6 +1,6 @@
 # Architecture in one paragraph
 
-F∆I has three concepts you'll meet everywhere: **Hub**,
+Ch∆In has three concepts you'll meet everywhere: **Hub**,
 **Module**, **Flow**.
 
 **The hub** is a single Rust binary called `fai`. It runs as a
@@ -39,7 +39,7 @@ Studio ──gRPC──▶ Hub ──loads──▶ Module (WASM)
   or Podman. Modules are WASM components loaded in-process.
 - No external database. SQLite ships with the hub.
 - No background services. One process; restart with
-  `fai daemon restart` — see Doctor → Daemon control.
+  `chain daemon restart` — see Doctor → Daemon control.
 
 ## Where to go next in this app