feat(date-time-picker): expose v-model:menu-open

[kelsos]

Summary

• Adds v-model:menu-open on RuiDateTimePicker that mirrors isOpen || calendarMenuOpen — true whenever any of the picker's teleported overlays (its own menu or the calendar's month/year sub-menu) is open.
• Lets parent RuiMenu/RuiDialog wrappers bind :persistent only for that window instead of forcing it on always.
• Storybook story InsideParentMenu and an example-app fixture demonstrate the pattern.

Closes #516. Unblocks rotki/rotki#12091 (CalendarDateNavigator Go-to-date dropdown can drop its manual :persistent + ESC workaround).

Test plan

• Unit: RuiDateTimePicker.spec.ts covers picker-menu open/close emit and "stays open while sub-menu opens after picker is open".
• E2E: apps/example/e2e/datetimepicker.spec.ts covers (1) parent stays open while only picker menu open, (2) parent stays open while sub-menu open, (3) parent closes when no picker overlay is open.
• pnpm run typecheck / pnpm run lint / pnpm run test:run (1072 pass) / pnpm run test:e2e (3 new + existing pass) / pnpm run build:prod green.
• web-types.json exposes menu-open + update:menuOpen.

Follow-up

A lu-calendar-days runtime warning shows up in the example app because the icon plugin auto-detects only consumer source. The picker stack's icons should be registered library-side (build-time emit + plugin baseline) so every consumer gets them for free. Will be a separate PR.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.89%. Comparing base (607759e) to head (7bc9bb8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #522      +/-   ##
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+ Coverage   84.88%   84.89%   +0.01%     
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  Files         143      143              
  Lines        5213     5217       +4     
  Branches     1553     1554       +1     
==========================================
+ Hits         4425     4429       +4     
  Misses        788      788              

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