Add monthly category spending visuals

[Dustin-ONeil]

Summary

Adds lightweight monthly spending visuals to the Stats page:

• Monthly stacked spending by category/category group
• Range category breakdown
• Balance timeline showing participant balances and reimbursements over time

The charts use existing expense/category data and custom HTML/CSS/SVG rendering. No chart dependency, persisted settings, Prisma fields, or migrations are added.

Details

• Adds category/group filtering controls, range controls, chart orientation controls, and rounded amount display.
• Excludes reimbursements from category spending totals.
• Uses UTC date parts for month/day grouping to avoid DATE timezone shifts.
• Adds a balance timeline with reimbursement markers, optional category markers, and stack-order control.
• Keeps visuals live through the existing stats query refresh path.

Testing

• pnpm test
• pnpm run check-types
• pnpm run lint
• targeted Prettier check on touched files

Transparency note

This is my third request using Codex for this kind of contribution. Happy to iterate to make sure it's up to standards

Reviewer Demo

Screenshots attached here. Any demo group/data should be fake and provided outside the deployed codebase.

• Monthly Spending is traditional stacked bar chart with expenses by category
• Balance Timeline is a stacked column chart showing outstanding balances ("Sam Owes" "Alex Is Owed") over the timeline of the group. I'm a data scientist (Masters in Statistics) so this is my best visual yet but I'm happy to iterate. I already went through hundreds of iterations to intuitively show the complexity of these interactions.