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/audit.md

@@ -37,7 +37,7 @@ WORM-1 is our first WORM (write-once-read-many) tier:
   beta / production channels.
 - **Tamper-evident**: the chain reveals any retroactive edit.
   Verification runs end-to-end on every Doctor-page load and
-  via `fai admin verify-events`.
+  via `chain admin verify-events`.
 - **Self-contained**: SQLite + chain hash is enough; no
   external WORM sink needed.
 
@@ -60,7 +60,7 @@ WORM sink) are roadmap items, not yet shipped.
 If you want to verify outside Studio:
 
 ```bash
-fai admin verify-events
+chain admin verify-events
 # Prints "Hash chain intact: N events verified" or names the
 # first event whose hash mismatches.
 ```