Add option to configure week start day (Monday) for welcome page heatmap

Problem

The activity heatmap on the Claude Code welcome page (What's up next, <user>?) is hardcoded to start the week on Sunday. This is the US convention, but most of the world (and the ISO 8601 standard) starts the week on Monday.

For users in regions where Monday-first is the norm (Europe, China, most of Asia), the heatmap layout feels off — the work week is split across rows, and the weekend isn't visually grouped.

Proposed Solution

Add a configuration option to control the first day of the week for the heatmap, e.g.:

• settings.json: "heatmapWeekStart": "monday" | "sunday" (default: "sunday" for backward compat)
• Or auto-detect from system locale (Intl.Locale().weekInfo.firstDay)

Why It Matters

• Aligns with ISO 8601 and the locale conventions of a large share of users
• Tiny UX win, but the welcome page is the first thing users see every session
• No breaking change if defaulted to current behavior