fix(edit): stop cursor jumping in event text fields

The event form's state pipeline ran .flowOn(io) over the whole combine,
including the _form round-trip every keystroke depends on. That async hop
handed BasicTextField a lagging value while typing, so Compose kept
correcting the cursor to the stale position.

Scope flowOn(io) to just the calendar/prefs/settings reads and collect the
form -> state -> UI path on the main dispatcher, so keystrokes round-trip
synchronously and the cursor stays put.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -7,6 +7,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+- Typing in the event title, location, and description fields no longer
+  makes the cursor jump around: the form state's round-trip to the UI was
+  hopping to a background dispatcher, so the text field saw a lagging value
+  while typing. Only the calendar/preferences reads stay off the main
+  thread now; the keystroke path is synchronous again
+
 ## [2.0.0] — 2026-06-11
 
 ### Added