fix(chart): don't render bars without a computed layout position

[dalley]

Fixes #30

Problem

A task that never received a computed layout position still renders a wx-bar element, producing a confusing "ghost bar":

• When date normalization is skipped — a task with unscheduled: true, or any task without a start date — the layout math leaves $x/$w as NaN.
• taskStyle() then emits left: "NaNpx", width: "NaNpx"; the browser rejects the invalid declarations.
• The bar element ends up with only top/height, so it sits at the chart origin and auto-sizes to its label text. To a user this reads as a real (and very wrong) schedule — the bar's apparent "duration" is the pixel length of its own title.

Repro

Add a dateless task to any demo dataset, e.g. in demos/data.js:

{ id: 999, text: 'Dateless task (ghost bar repro)', parent: 0 },

A bar appears at the chart start spanning roughly six "days" — the width of the label.

This is easy to hit from the MIT package: unscheduledTasks is disabled in the store's init(), but normalizeDates still honors the per-task unscheduled flag (and calcDates is a no-op for tasks with no start), so any dateless task paints a ghost bar with no way to opt out.

Fix

Guard the main bar and the baseline element on Number.isFinite() of their computed positions ($x / $x_base). The row still appears in the grid; nothing is painted on the timeline until the task has real coordinates.

!= null is not sufficient — the skipped-normalization path produces NaN, not undefined.

Verification

Ran the demos with the repro task above: before — ghost bar at the origin; after — the grid row is present and the timeline row is empty, while all scheduled bars, milestones, summaries and baselines render unchanged.

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