fix(cycle): gate the calendar grid to a learning state during cold start

The predictions panel suppressed its fertile-window and ovulation rows while
fewer than three cycles were observed, but the month calendar did not — so a
brand-new user who logged a single period saw the grid paint a confident
fertile-window shading, a predicted-ovulation dot, and full phase bands off a
population 28/14 prior at ~0.20 confidence. Calendar-based fertility estimation
is unreliable below several recorded cycles, so the grid was asserting days it
could not know.

Thread the same still-learning gate into the calendar build: while fewer than
three cycles are observed, suppress the fertile window, the ovulation dot, and
the phase band (the predicted-period bar stays, mirroring the panel). Surface a
top-level stillLearning flag on the calendar response so the client renders a
calm "learning your cycle" banner over the grid instead of a bare, falsely
confident calendar. The flag is additive and back-compatible; older clients
ignore it and degrade to the previous render.

Also pin the observed-cycle count to a single source — the engine's
post-exclusion value — so the response never carries two divergent counts.

docs/api/openapi.yaml

@@ -6608,6 +6608,8 @@ components:
               anyOf:
                 - $ref: "#/components/schemas/CyclePredictionDTO"
                 - type: "null"
+            stillLearning:
+              type: boolean
             days:
               type: array
               items:
@@ -6691,6 +6693,7 @@ components:
           required:
             - profile
             - prediction
+            - stillLearning
             - days
             - meta
           additionalProperties: false