refactor(planning): give planning calculations one state owner

PlanningCalculationService now builds the submitted request, owns the
pending and accepted calculation state, and delivers explanations for
the accepted calculation only. The duration estimator reads the same
request. The calendar and leave-balance summary consume the accepted
outcome instead of coordinating writable state on the input service.

Refs #275

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/planning-duration.md

@@ -73,9 +73,8 @@ range is explicitly indicative, not a confidence interval or a promise.
 
 ## What is timed
 
-Manual requests capture `performance.now()` inside
-`UserInputService.requestComputation`, before updating the calculation trigger.
-The existing initial calculation falls back to the start of `startCalculation`.
+Every request, manual or initial, captures `performance.now()` inside
+`PlanningCalculationService.start`.
 The endpoint is Angular's `afterNextRender` callback after the complete plan has
 been projected into the calendar, its months and placements have been rendered,
 and the loading overlay has been removed. This measures request scheduling,