feat(session-loading): show progress for cloud and local sessions

[Harry19081]

Problem / Motivation

Opening a session can involve downloading, copying, or hydrating a meaningful amount of data. Today, cloud and local session loading can appear stalled because users cannot see whether work is progressing or how much remains.

Proposed Solution

Show a progress indicator while a session is being prepared, for both cloud-hosted and local sessions.

The UI should:

• Display a determinate progress bar when total work or byte size is known.
• Fall back to an indeterminate progress state when a total cannot be calculated.
• Describe the current phase, such as fetching metadata, downloading files, loading local data, or finalizing the session.
• Show transferred and total size when available.
• Surface failures clearly and offer retry where recovery is possible.

Alternatives Considered

A generic loading spinner provides activity feedback but does not tell users how much work remains, especially for large sessions.

Acceptance Criteria

• Cloud session downloads show visible progress from start through completion.
• Local session loading shows visible progress for any non-trivial preparation work.
• Known totals use a determinate progress bar; unknown totals use an indeterminate state.
• The current loading phase is communicated in user-facing text.
• Transfer size or item counts are shown when available.
• Loading errors are surfaced without leaving the UI in a permanent loading state.
• The session opens normally after preparation completes.

Additional Context

This issue covers progress visibility. A separate issue covers asking for confirmation before downloading unusually large sessions.