Add event funnels

[tudddorrr]

Event funnels

• Add event funnel entity, API hooks, and validated request/response helpers for create, update, delete, and refresh operations.
• Add Event Funnels list, new, and edit pages plus an empty-state when a game has no funnels yet.

Funnel builder and preview

• Add a builder UI to define a funnel name, max gap, and an ordered sequence of 2–5 steps, each with prop filters using and/or rule modes and comparison operators.
• Add a live preview that runs the configured steps over a selected date range, with a debounced fetch, result bar chart, and manual refresh of saved funnels.

Navigation and routing

• Add /events/funnels routes and wire Event funnels into the Events secondary nav alongside Events overview.
• Make SecondaryNav highlight the longest matching route so nested subpages stay highlighted.

Validation and helpers

• Add funnel-rules utilities enforcing step count, distinct step names, and per-rule operand arity, covered by unit tests.
• Add a white-small Button variant and refine TextInput error highlighting for funnel forms.