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>

lib/widgets/fai_card.dart

@@ -1,5 +1,5 @@
 // FaiCard — flat card with subtle accent gradient on the top
-// border. Replaces Material's default Card for the F∆I look.
+// border. Replaces Material's default Card for the Ch∆In look.
 
 import 'package:flutter/material.dart';
 

lib/widgets/fai_delta_mark.dart

@@ -1,4 +1,4 @@
-// FaiDeltaMark — the F∆I signature element. A precise triangle
+// FaiDeltaMark — the Ch∆In signature element. A precise triangle
 // (∆) drawn from primitives, not a font glyph. Three states:
 //
 // - idle: static, accent colour, soft glow

lib/widgets/fai_empty_state.dart

@@ -2,7 +2,7 @@
 // Replaces "(no data)" strings with one tone, one icon, one tip.
 //
 // The blank surface is where an app most easily feels unfinished,
-// so this state is deliberately *designed*: the F∆I delta (∆)
+// so this state is deliberately *designed*: the Ch∆In delta (∆)
 // sits as a soft watermark behind the icon badge, the badge
 // carries a whisper of elevation, and the copy breathes. The
 // result reads "nothing here yet, on purpose" rather than
@@ -48,7 +48,7 @@ class FaiEmptyState extends StatelessWidget {
               // Brand hero: an oversized, low-opacity ∆ watermark
               // anchors the composition while the contextual icon
               // badge sits crisply on top — so every empty surface
-              // still carries the F∆I signature, never a bare icon.
+              // still carries the Ch∆In signature, never a bare icon.
               SizedBox(
                 width: 112,
                 height: 112,

lib/widgets/fai_settings_dialog.dart

@@ -1013,7 +1013,7 @@ class McpClientDraft {
 /// MCP-clients panel — shows every configured server with its
 /// last-discovery health, plus Add / Remove / Refresh actions.
 /// Configure a server here, the store gains synthetic
-/// capabilities for every advertised tool. Mirrors `fai mcp …`
+/// capabilities for every advertised tool. Mirrors `chain mcp …`
 /// on the CLI.
 class _McpClientsPanel extends StatelessWidget {
   final List<McpClientInfo> clients;
@@ -1669,7 +1669,7 @@ class _ThemePluginPanel extends StatelessWidget {
 }
 
 /// Hub maintenance panel — currently one action: reset operator
-/// state via `fai reset --yes`. Lives at the bottom of Settings
+/// state via `chain reset --yes`. Lives at the bottom of Settings
 /// so the everyday config controls aren't crowded by a destructive
 /// button. The reset itself is gated by an interactive confirm
 /// dialog before the CLI is invoked.

lib/widgets/fai_system_ai_editor.dart

@@ -820,7 +820,7 @@ class _ModelPicker extends StatelessWidget {
 /// round-trip; works for Ollama tags (`gemma3:4b`), OpenAI names
 /// (`gpt-4o-mini`), Anthropic-style (`claude-haiku-4-5`).
 enum _Suitability {
-  recommended, // tested with F∆I System AI
+  recommended, // tested with Ch∆In System AI
   balanced, // 4-8B params or known-good cloud model
   small, // <4B params; quality may be insufficient
   large, // 9-15B; slower but ok

lib/widgets/widgets.dart

@@ -1,4 +1,4 @@
-// Barrel file for the F∆I Studio widget library.
+// Barrel file for the Ch∆In Studio widget library.
 //
 // Pages should `import '../widgets/widgets.dart';` and use the
 // primitives directly — no Material `Card` / generic `Container`