Persist mass chart slider state to URL parameters

[Copilot]

The ship mass over time sliders on the 'constant accel' and 'flip' tabs were not persisted to the URL, preventing users from sharing specific chart views.

Changes

• URL state configuration: Added massSlider parameter to motion and flip tab configs, enabling automatic URL sync through existing infrastructure
• Deferred restoration: Implemented applyPendingSliderValue() to restore slider values after chart initialization, when slider ranges are known
• Chart integration: Updated accel/flip handlers in main.ts to apply pending URL values post-initialization

Implementation

The slider values are synced bidirectionally:

• Write path: Existing setupURLSync() automatically captures slider changes
• Read path: New mechanism stores URL values as pending, then applies them after chart/slider initialization completes

// Store pending value from URL during initial load
if (paramName === 'massSlider') {
    const numValue = parseFloat(paramValue);
    if (!isNaN(numValue)) {
        pendingSliderValues[inputId] = numValue;
    }
}

// Apply after chart initialization (50ms delay for async chart rendering)
setTimeout(() => {
    initializeMassChartSlider(...);
    applyPendingSliderValue('accelMassSlider', 'accelMassSliderValue', 'days', 'accelMassChart', chartRegistry);
}, 50);

Range validation ensures slider values remain within chart data bounds, with error handling for chart update failures.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Slider state management</issue_title>
<issue_description>In the JavaScript project, the ‘constant accel’ and ‘flip’ tabs have diagrams for ship mass over time. This has a slider to control the x axis, but it is not persisted on the URL. Let’s fix that</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Slider state management #63

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