Fix duplicate preview default filters and empty-state messaging

[Greensand321]

Motivation

• The preview HTML could display groups in the overview but show a misleading "No changes needed." message because filter defaults hid ready/no-op groups.
• The intent is to make previews more informative when there are no actionable changes.
• Provide clearer empty-state messaging so users understand when there are simply no duplicate groups versus no planned actions.

Description

• Compute actionable_groups ahead of rendering controls and set showReady/showNoOps defaults to checked when there are zero actionable changes.
• Render the showReady and showNoOps checkboxes with checked state using the show_ready_default and show_no_ops_default flags.
• Replace the single ambiguous message "No changes needed." with two clearer messages: "No duplicate groups found." and "No planned changes. All groups are ready or no-op.".
• Consolidate the actionable-group counting so it’s not double-counted during group rendering and moved before control rendering in duplicate_consolidation.py.

Testing

• No automated tests were run for this change.

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Codex Task