fix: implement hard limit for canvas width when using forecast mode

[troinine]

Fixes #40

[Copilot]

Pull request overview

This PR implements a hard limit on canvas width to prevent rendering errors when displaying large forecast datasets in chart mode. The implementation addresses browser canvas size limitations by truncating forecast data to fit within a 16384-pixel width constraint.

Key Changes:

• Introduces a MAX_CANVAS_WIDTH constant (16384px) and a safeForecast getter that truncates forecast data when necessary
• Updates all chart-related methods to use the truncated forecast data instead of the raw forecast property
• Adds comprehensive test coverage for the truncation logic with a 500-item forecast scenario

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/components/wfc-forecast-chart.ts	Implements canvas width limiting by adding MAX_CANVAS_WIDTH constant, safeForecast getter with gap-aware calculation, and updates render/chart methods to use truncated data
test/weather-forecast-chart.test.ts	Adds test case verifying 500-item forecast is correctly truncated to 327 items based on itemWidth=50 and gap=0
README.md	Documents the 16384px canvas width limitation and explains that forecast data exceeding this limit will be truncated

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