agent: mask OAuth code input + reveal toggle on login screen (closes #568)

The login-in-progress screen's OAuth-code input was a plain text
field — anyone shoulder-surfing or capturing a screenshot of the
agent web UI would see the code in cleartext. Same risk applies
to dashboard share-screens during live demos.

Changes:

- input switches to type='password' so the pasted code renders
  as bullets by default. Placeholder updated to '(hidden)' so the
  operator knows the masking is intentional, not a browser quirk.
- new 'reveal' button (👁) next to the input flips the type back
  to text on press, so the operator can sanity-check the paste
  before submitting if she wants. aria-pressed reflects state.
- CSS for the reveal button mirrors the existing .btn-login amber
  family — quiet by default, amber border/glow when pressed.
- spellcheck='false' on the input so browsers don't try to
  underline the random-looking string as a typo.

The on-screen OAuth URL stays visible (the operator needs to
click it). The code is the secret leg — only the operator's
browser holds it, the URL is what was posted publicly to claude's
OAuth provider.

frontend/packages/agent/src/agent.css

@@ -413,7 +413,7 @@ a:hover { color: var(--fg); text-shadow: 0 0 12px rgba(137, 220, 235, 0.9); }
   flex: 1;
 }
 .sendform input:focus { outline: 1px solid var(--purple); }
-.loginform { display: flex; gap: 0.6em; margin-top: 0.5em; }
+.loginform { display: flex; gap: 0.6em; margin-top: 0.5em; align-items: stretch; }
 .loginform input {
   font-family: inherit; font-size: 1em;
   background: rgba(255, 255, 255, 0.04);
@@ -423,6 +423,36 @@ a:hover { color: var(--fg); text-shadow: 0 0 12px rgba(137, 220, 235, 0.9); }
   flex: 1;
 }
 .loginform input:focus { outline: 1px solid var(--purple); }
+
+/* #568: show / hide toggle for the masked OAuth-code input. Quiet
+   by default (muted border + transparent bg), lights amber on
+   hover / when pressed (aria-pressed="true") so the operator
+   sees at a glance whether the code is currently visible. */
+.loginform-reveal {
+  font-family: inherit;
+  font-size: 1em;
+  background: transparent;
+  color: var(--muted);
+  border: 1px solid var(--purple-dim);
+  border-radius: 3px;
+  padding: 0 0.6em;
+  cursor: pointer;
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  transition: color 0.15s ease, border-color 0.15s ease, box-shadow 0.15s ease;
+}
+.loginform-reveal:hover,
+.loginform-reveal:focus-visible {
+  color: var(--amber);
+  border-color: var(--amber);
+  outline: none;
+}
+.loginform-reveal[aria-pressed="true"] {
+  color: var(--amber);
+  border-color: var(--amber);
+  box-shadow: 0 0 8px -2px var(--amber);
+}
 pre.diff {
   background: rgba(255, 255, 255, 0.03);
   border: 1px solid var(--purple-dim);

frontend/packages/agent/src/app.js

@@ -351,11 +351,33 @@ window.marked = marked;
       const code = el('form', {
         action: '/login/code', method: 'POST', class: 'loginform', 'data-async': '',
       });
+      // #568: OAuth code is a sensitive secret — mask the input with
+      // type="password" so a shoulder-surfer / screenshot doesn't
+      // capture it. The reveal button flips it back to text on press
+      // so the operator can sanity-check the paste before submit.
+      const codeInput = el('input', {
+        name: 'code',
+        type: 'password',
+        placeholder: 'paste OAuth code here (hidden)',
+        required: '',
+        autocomplete: 'off',
+        spellcheck: 'false',
+      });
+      const reveal = el('button', {
+        type: 'button',
+        class: 'loginform-reveal',
+        title: 'show / hide pasted code',
+        'aria-label': 'show / hide pasted OAuth code',
+        'aria-pressed': 'false',
+      }, '👁');
+      reveal.addEventListener('click', () => {
+        const showing = codeInput.getAttribute('type') === 'text';
+        codeInput.setAttribute('type', showing ? 'password' : 'text');
+        reveal.setAttribute('aria-pressed', showing ? 'false' : 'true');
+      });
       code.append(
-        el('input', {
-          name: 'code', placeholder: 'paste OAuth code here',
-          required: '', autocomplete: 'off',
-        }),
+        codeInput,
+        reveal,
         el('button', { type: 'submit', class: 'btn btn-login' }, '◆ S3ND C0DE'),
       );
       root.append(code);