[FEATURE] Copy a range of consecutive debugger steps

# Feature

Currently, users can only copy debugger steps individually via the "Copy step" button on each step card. It would be helpful to select and copy a range of consecutive steps, so users can easily share only the relevant part of the flow with Claude or an engineer (rather than copying the whole log or one step at a time).

This applies to both the **OAuth Debugger** and the **XAA Debugger**, which both have per-step "Copy step" buttons on their step cards.

# Solution

Add a "Select" mode toggle near the existing "Copy" button on the Guide view. In select mode:
- Each step card gets a checkbox.
- Clicking a step selects it; shift-clicking selects everything between the last-clicked step and the current one (standard range-select UX).
- With one or more steps selected, a "Copy N steps" action copies the selected range using the same sanitized text format as the existing single-step/full-log copy (redacting tokens, secrets, cookies, etc.).

# Alternatives

- Multi-select via checkboxes without shift-click range support (more clicks for long ranges).
- A "start step" / "end step" picker instead of inline checkboxes.

# Additional Context

Today, each step card in the OAuth Debugger (and similarly the XAA Debugger) has an individual "Copy step" button — see step 7 ("Client Credentials Received") and step 8 ("Generate PKCE Parameters") in the Guide view. There's no way to select steps 7 and 8 together and copy both in one action.

Start in the Guide view by tracing the existing per-step "Copy step" behavior for the OAuth Debugger and XAA Debugger. Add the select-mode, checkbox, shift-range, and "Copy N steps" interactions while preserving the existing sanitized text format; done means consecutive selected steps copy together without exposing tokens, secrets, or cookies.

Written by the indexing model from the issue text.