fix(xai): accept Grok Build code during loopback wait + tiny screenshot guard

xAI's consent page renders the authorization code in-page instead of
redirecting to the loopback callback, so the listener just hangs and the
manual-paste flow demands a callback URL that never contains the token.

- auth.py: poll stdin non-blockingly while waiting for the xAI loopback
  callback; accept a pasted bare Grok Build code and substitute the locally
  generated state (PKCE code_verifier still binds the exchange). No need to
  wait for timeout or re-run with --manual-paste.
- computer_use: parse PNG/JPEG dimensions from base64 and fall back to the
  text/AX/SOM payload when the screenshot is below the provider minimum
  (8x8), which xAI rejects with HTTP 400.
- model_setup_flows.py: xAI credential reuse prompt uses the standard radio
  picker via a shared _prompt_auth_credentials_choice helper.
- main.py: thread a title through _prompt_provider_choice; re-home the helper
  import (flows live in model_setup_flows.py post-decomposition).

Salvaged from #36781 onto current main (contributor's main.py edits re-homed
to model_setup_flows.py, where the flows were extracted since the PR opened).

hermes_cli/main.py

@@ -499,6 +499,7 @@ from hermes_cli import __version__, __release_date__
 # (god-file decomposition Phase 2). Re-imported here so select_provider_and_model and
 # existing test monkeypatches (hermes_cli.main._model_flow_*) keep resolving unchanged.
 from hermes_cli.model_setup_flows import (
+    _prompt_auth_credentials_choice,
     _model_flow_openrouter,
     _model_flow_nous,
     _model_flow_openai_codex,
@@ -2830,7 +2831,12 @@ def select_provider_and_model(args=None):
         member_labels = [
             provider_labels.get(m, m) for m in selected_members
         ]
-        member_idx = _prompt_provider_choice(member_labels, default=member_default)
+        group_label = ordered[provider_idx][1].split(" ▸", 1)[0]
+        member_idx = _prompt_provider_choice(
+            member_labels,
+            default=member_default,
+            title=f"Select {group_label} provider:",
+        )
         if member_idx is None:
             print("No change.")
             return
@@ -3331,7 +3337,7 @@ def _aux_flow_custom_endpoint(task: str, task_cfg: dict) -> None:
     print(f"{display_name}: custom ({short_url})" + (f" · {model}" if model else ""))
 
 
-def _prompt_provider_choice(choices, *, default=0):
+def _prompt_provider_choice(choices, *, default=0, title="Select provider:"):
     """Show provider selection menu with curses arrow-key navigation.
 
     Falls back to a numbered list when curses is unavailable (e.g. piped
@@ -3341,7 +3347,7 @@ def _prompt_provider_choice(choices, *, default=0):
     try:
         from hermes_cli.setup import _curses_prompt_choice
 
-        idx = _curses_prompt_choice("Select provider:", choices, default)
+        idx = _curses_prompt_choice(title, choices, default)
         if idx >= 0:
             print()
             return idx
@@ -3349,7 +3355,7 @@ def _prompt_provider_choice(choices, *, default=0):
         pass
 
     # Fallback: numbered list
-    print("Select provider:")
+    print(title)
     for i, c in enumerate(choices, 1):
         marker = "→" if i - 1 == default else " "
         print(f"  {marker} {i}. {c}")