Fix Progress component

[clarasb]

Description

This PR adds visible support for Chartlets progress indicators and connects it to callback execution state.

Progress components can now be hidden with visible=False and shown via callback outputs such as:

Output("progress", "visible")

When a server-side callback declares a progress component’s visible property as an output, chartlets now shows that progress indicator immediately while the callback request is pending. Once the callback completes, the normal callback output is applied, typically hiding the progress component again.

Changes

• Added visible to the shared component state model.
• Made CircularProgress and LinearProgress respect visible={false}.
• Added pending-callback progress handling in invokeCallbacks().
• Added protection for overlapping pending callbacks so a progress component is not hidden too early.
• Added/updated tests for progress visibility and Python serialization.
• Added a demo panel.

Demo Panel

[codecov]

Codecov Report

❌ Patch coverage is 91.54930% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.15%. Comparing base (f8cf975) to head (13ca6d6).

Files with missing lines	Patch %	Lines
...ackages/lib/src/actions/helpers/invokeCallbacks.ts	90.47%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #145      +/-   ##
==========================================
+ Coverage   92.53%   93.15%   +0.61%     
==========================================
  Files          90       90              
  Lines        1594     1665      +71     
  Branches      243      262      +19     
==========================================
+ Hits         1475     1551      +76     
+ Misses         99       98       -1     
+ Partials       20       16       -4     

Flag	Coverage Δ
backend	100.00% <100.00%> (ø)
frontend	86.89% <91.30%> (+1.75%)	⬆️

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