fix(dashboard): profile-scope Channels endpoints and seed per-profile .env (#44792)

Two halves of the same community report (dashboard Profile Builder):

1. A fresh dashboard/CLI-created profile got no .env file unless cloned,
   so it silently inherited API keys and messaging tokens from the shell
   environment / root install. create_profile() now seeds a placeholder
   .env (0600) for non-clone profiles, matching the SOUL.md seeding.

2. The Channels endpoints (/api/messaging/platforms GET/PUT/test) were
   not profile-scoped: they read/wrote the dashboard process's own .env
   via load_env()/save_env_value() regardless of the global profile
   switcher. They now accept the standard optional profile param (body
   beats query on the PUT, matching other scoped writes) and run inside
   _profile_scope(). When scoped, the payload no longer falls back to
   os.environ or load_gateway_config()'s env-override layer — both carry
   the ROOT install's credentials and would misreport them as the
   profile's. /api/messaging/platforms added to PROFILE_SCOPED_PREFIXES
   so the sidebar switcher scopes the Channels page automatically.

web/src/lib/api.ts

@@ -60,14 +60,16 @@ export function getManagementProfile(): string {
 
 // Endpoint families that honor ?profile= on the backend (web_server.py
 // _profile_scope). Anything else — sessions, analytics, ops, pairing,
-// channels, cron (which has its own per-job profile params), profiles
-// themselves — is machine-global or self-scoped and must NOT be rewritten.
+// telegram onboarding, cron (which has its own per-job profile params),
+// profiles themselves — is machine-global or self-scoped and must NOT be
+// rewritten.
 const PROFILE_SCOPED_PREFIXES = [
   "/api/skills",
   "/api/tools/toolsets",
   "/api/config",
   "/api/env",
   "/api/mcp",
+  "/api/messaging/platforms",
   "/api/model/info",
   "/api/model/set",
   "/api/model/auxiliary",