docs: correct the "default password123" claim — production nodes have none

The walkthrough told new users to log in with `password123` and said they'd
be "prompted to change this password immediately". Neither is true on a
release build:

- `AuthManager::ensure_default_user` is never called. `main.rs:356-362`
  says so explicitly ("Don't auto-create default user — let onboarding flow
  handle password setup via auth.setup"), and the function is `#[allow(dead_code)]`.
- The only `password123` login path is `api/rpc/auth.rs:36-46`, which is
  `#[cfg(debug_assertions)]` AND `dev_mode` AND only fires *before* setup —
  no release binary carries it.
- `Login.vue` calls `auth.isSetup` on mount and renders the "Set Up Your
  Node" password-creation form when it returns false. That is the real
  first-boot screen, and it is the only `auth.setup` caller in the frontend.

So there is nothing to be "prompted to change" — the user creates the
password themselves, and the doc's version taught them to look for a
default that does not exist.

Fixed in four places:
- user-walkthrough Step 8 rewritten as "Create Your Password"
- troubleshooting's "Default password is password123" solution replaced,
  including the warning that deleting user.json does NOT recover a lost
  password (the onboarding gate refuses auth.setup on a provisioned node)
- api-reference cURL example uses a placeholder, not the fake default
- 1.8.0 hardening plan's "kill default credentials" item now reflects that
  the web half is done and only the SSH defaults still ship

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/user-walkthrough.md

@@ -91,13 +91,21 @@ The auto-installer handles everything:
 2. Tap or click anywhere to proceed
 3. A typing animation welcomes you: "Welcome, Noderunner"
 
-### Step 8: Login Screen
+### Step 8: Create Your Password
 
-> **Screenshot**: The login screen with a password field and glass-morphism design.
+> **Screenshot**: The "Set Up Your Node" screen with password and confirm-password fields, glass-morphism design.
 
-1. Enter the default password: `password123`
-2. Click "Login"
-3. You'll be prompted to change this password immediately
+**There is no default web password.** A freshly installed node has no user
+account at all, so this screen shows a password-creation form rather than a
+login form:
+
+1. Enter a password (minimum 8 characters)
+2. Confirm it in the second field
+3. Click "Set Up Node"
+
+Every boot after this one shows the normal login form and asks for the password
+you chose here. Store it somewhere you can get back to — recovering it requires
+SSH access to the node.
 
 ### Step 9: Choose Your Path (Onboarding)