feat(FN-2695): gate Todos view behind todoView experimental flag

- Add todoView to experimental feature settings and expose it in Settings modal and docs
- Derive todosEnabled in app settings hook with regression tests for enabled/disabled behavior
- Gate App routing so Todos only renders when the flag is enabled and redirects otherwise
- Hide Todos entries in header overflow and mobile navigation when the flag is off

docs/settings-reference.md

@@ -666,6 +666,7 @@ Common built-in dashboard flags include:
 - `skillsView`
 - `nodesView`
 - `devServerView`
+- `todoView`
 
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