fix(chart): defer sankey render until container has a non-zero size

[TomK]

Problem

A zn-chart type="sankey" crashes on draw with:

TypeError: Cannot read properties of null (reading '0')
    at ... r._updateViewCoordSys ... r.render

The crash is not caused by the chart data (orphan nodes, link values, etc.). The trigger is a zero-width or zero-height container at render time. ECharts' bar/line layouts tolerate a zero-size container; the sankey layout does not — it builds a degenerate coordinate system and reads null[0] in _updateViewCoordSys, matching the reported stack trace.

zn-chart forces the host height (default 300px) but takes its width from layout. So when a sankey chart initialises inside a hidden tab or a flex/grid cell that has not been laid out yet, width is 0 and it crashes.

Fix

Defer setOption until the chart container reports a non-zero size:

• New renderChart() guard — if width or height is 0, mark pendingRender and skip; otherwise apply the option.
• The existing ResizeController triggers the deferred render once the container gains a size (e.g. a hidden tab becomes visible).
• All full-option setOption call sites (init, live updates, attribute changes) routed through the guard.

No behaviour change for charts that already have a sized container.

Test plan

• Reproduced the exact error in a real headless browser (Playwright + echarts canvas): a sankey chart set into a width:0 / height:300px container threw Cannot read properties of null (reading '0'); bar and line charts in the same container rendered fine.
• Verified the fixed control flow in the same harness: initialising at width:0 defers without throwing (pendingRender=true), and after the container is given a width the ResizeObserver fires and the chart renders (series.length > 0) with no page errors.
• Added a regression test in chart.test.ts (does not crash when a sankey chart starts in a zero-width container).
• tsc --noEmit passes; no new lint errors introduced.

Notes for reviewer

• The repo's component test suite imports the built dist/zn.min.js, so the new regression test only exercises this fix after a dist rebuild. I did not rebuild dist in this PR because the production build currently fails on an unrelated, pre-existing error (markdown-editor.component.ts(282,53): Property 'description' does not exist on type 'ZnMarkdownEditor'). That should be addressed separately before the next bundle is cut.

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