feat(tui): add tab completions for /login mode subcommand

Implement tab completion for /login <provider> mode <value> syntax,
allowing users to easily switch auth modes (api_key, oauth, auto) with
keyboard navigation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/frontends/tui/commands.test.ts

@@ -245,6 +245,18 @@ describe('getCommandCompletions', () => {
     const completions = getCommandCompletions('/ru');
     expect(completions).toContain('/runtime');
   });
+
+  it('completes /login <provider> mode values', () => {
+    const completions = getCommandCompletions('/login anthropic mode ');
+    expect(completions).toContain('/login anthropic mode api_key');
+    expect(completions).toContain('/login anthropic mode oauth');
+    expect(completions).toContain('/login anthropic mode auto');
+  });
+
+  it('filters mode completions by partial input', () => {
+    const completions = getCommandCompletions('/login anthropic mode o');
+    expect(completions).toEqual(['/login anthropic mode oauth']);
+  });
 });
 
 describe('isToolInventoryQuery', () => {

src/frontends/tui/commands.ts

@@ -368,6 +368,20 @@ export function getCommandCompletions(partial: string): string[] {
     return [];
   }
 
+  // Complete /login <provider> mode <value>
+  if (trimmed.startsWith('/login ')) {
+    const rest = trimmed.slice('/login '.length).trim();
+    const parts = rest.split(/\s+/);
+    // /login <provider> mode <partial>
+    if (parts.length >= 2 && parts[1] === 'mode') {
+      const partial = (parts[2] ?? '').toLowerCase();
+      const modes = ['api_key', 'oauth', 'auto'];
+      return modes
+        .filter(m => m.startsWith(partial))
+        .map(m => `/login ${parts[0]} mode ${m}`);
+    }
+  }
+
   // Complete /model <tier> <provider/model>
   if (trimmed.startsWith('/model ')) {
     const args = trimmed.slice('/model '.length).trim();