docs(context): define today as the Europe/Paris calendar date

The today marker and past styling follow the same Paris date that
starts the default planning horizon (#301).

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

CONTEXT.md

@@ -24,6 +24,10 @@ The unspent portion of a leave balance that expires after the planning horizon.
 The category of leave in a balance or on a declared non-working day: CP (paid leave), RTT (working-time reduction leave), or Other (a user-managed catch-all, including any other reason for not working).
 _Avoid_: ANY as a leave type; `ANY` is only a rule selector.
 
+**Today**:
+The current Europe/Paris calendar date, whatever the device's time zone. The calendar marks it as today and shows earlier dates as past.
+_Avoid_: local date
+
 **Planning horizon**:
 The user-selected half-open date interval within which leave may be placed; its start may precede today's Europe/Paris calendar date and its duration may exceed twelve months. It defaults to today through the same date twelve months later, and explicitly selected boundaries remain unchanged as time passes, with past dates still eligible for leave placements.