refactor(web): unify floating event forms

[tyler-dane]

Summary

• replace Day's module-level floating stores with a local Floating UI controller shared by Day and Week
• render both calendar forms through one floating shell with declarative reference refs
• give the Day context menu independent virtual positioning and remove timer/DOM lookup reopening logic

Why

Day and Week had separate form-positioning lifecycles, while Day also coordinated form and context-menu state through five external stores. This makes open/close behavior local, explicit, and consistent with Floating UI's controlled-state and ref APIs.

Validation

• bun test:web — 1,050 tests passed
• bun type-check
• bun lint — passes with 13 pre-existing warnings
• React Doctor changed-file scan — 91/100, no issues found