Fix legacy container restoration after dependency upgrade

[Copilot]

Containers created before the Axum upgrade couldn't reconnect because the restoration logic required session_id labels that didn't exist on pre-upgrade containers.

Changes

Legacy Container Restoration

• Extract UUID from container name patterns (trycli-studio-viewer-{uuid}, trycli-studio-session-{uuid}) and use as session_id for containers without labels
• Add name-based fallback matching in restore_specific_session() when label-based lookup fails

Prevent Duplicate Sessions

• Add find_existing_viewer_container() to check for running containers before get_project() creates new session IDs
• Reuse existing container session_id instead of generating new UUID on every project access

Code Quality

• Extract calculate_session_created_at() helper to eliminate timestamp calculation duplication across 4 call sites

Example

Before:

// Only restored containers with session_id label
if let (Some(session_id), Some(names)) = (session_id, container.names) {
    // restore...
}

After:

// Handle both labeled and legacy containers
if let Some(session_id) = session_id {
    // restore with label
} else {
    // Extract UUID from name: trycli-studio-viewer-{uuid}
    let legacy_session_id = container_name.strip_prefix("trycli-studio-viewer-");
    // restore with extracted UUID
}

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