Add moving average option to statistics chart

[LuisMars]

Closes #889

Adds an optional moving average line to the statistics chart, smoothing the selected stat so it is easier to see whether it is trending up or down despite day-to-day noise. As raised in the issue thread, it applies to any stat, not just weight.

• "Show Moving Average" toggle and a configurable "Moving Average Period" (default 7) in Statistics settings
• Trailing simple moving average drawn as a second line over the chart, separate from the existing average annotation; ignores data gaps and works on both bar and line charts
• English strings added; other locales translated where possible, with English fallback

Tested in the browser build: Statistics settings -> enable Show Moving Average -> open Statistics -> pick a stat with history.

[davidhealey]

This looks ok. The only thing I can see is the language files. You should only add your strings to the en file. All the others are handled through Crowdin.

[LuisMars]

Fixed