Add reports dashboards with custom date ranges

[jmgasper]

Summary

https://topcoder.atlassian.net/browse/PM-5703

• add the Reports dashboard landing page and detail routes for signups, paid members, and challenge participation
• use vertical column charts for all three reports so every timeline appears on the bottom X-axis
• add an accessible custom month-range mode alongside the default six-month view
• keep chart requests, period navigation, and CSV exports aligned to the same UTC range
• retain the requested Challenge terminology and omit empty card info controls

Why

Testing showed that the horizontal charts in reports 2 and 3 made their timelines inconsistent with report 1, and users also need to inspect and export reporting periods outside the default six months.

Implementation notes

Custom month inputs are inclusive for users, while requests preserve the existing half-open API contract. For example, July 2025 through June 2026 is sent as 2025-07-01 through the exclusive boundary 2026-07-01. Previous and next navigation preserve the selected number of months, and future periods remain disabled.

This branch was created directly from origin/master. A historical dashboard-tweaks commit also contained unrelated Review-app work, so only its Reports dashboard paths were retained here.

Validation

• yarn test:no-watch --runInBand src/apps/reports/src — 11 suites, 45 tests passed
• yarn lint — passed
• yarn run build — completed successfully (existing repository CSS-order and lint warnings remain non-blocking)