Keep a repeating week on its weekday when the planner picks the dates

A yearly-repeating period kept the same calendar date instead of the same
weekday: 2026-10-05..10-11 (Mon-Sun) recurred as 2027-10-05..10-11 (Tue-Mon)
rather than 2027-10-04..10-10. The snap added in 224fe7a only ever covered
window_mode == 'fixed'; 'flexible' wishes were unaligned at both stages.

project_window now snaps both modes. A flexible bound is still a bounding
range, but it keeps its weekday alignment year over year with its span held
fixed, so the search space never shrinks and a bound that is exactly as long
as the target (its only candidate) repeats week-shaped.

_rank_candidate gains a Monday-start term between cost and start, so equal-cost
candidates break on week alignment instead of "earliest". It sits after cost
deliberately: a cheaper non-Monday start still wins, because red days outrank
alignment.

_reuse_already_funded_window would otherwise freeze a pre-existing misaligned
placement forever, so it now releases a year whose reuse winner is misaligned
when a fresh search ranks strictly better -- never when one of its days is
pinned or excluded, so a deliberate per-day override always survives.

Migration 008 realigns flexible series already materialised under the old rule;
projection runs once, at create/update, so nothing repairs them on read.

Alongside this, the planner now says when it moved a period to catch red days.
WindowOutcome carries shift_gain_days (the saving over the naive bound-start
placement) and the calendar toasts it, once, on the sync that decided it.
A fixed window is never auto-moved -- its "exact requested placement,
unconditionally" guarantee stands -- but its shift suggestion is no longer
hidden behind a shortfall, so a fully-funded one can surface a cheaper week too.

Editing a period in the popover now also sends window_mode: 'fixed' with a
matching target. Preset chips create flexible wishes; dragging one down to an
exact week left it flexible with a week-long bound, which is the degenerate
single-candidate case that produced the reported dates.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DXPBfr1v7F71t6QcbHCaaT

PLANNING_ENGINE_SPEC.md

@@ -624,7 +624,15 @@ class LeaveWish:
     # inside for the best-costing placement (e.g. "vår" = 03-01..05-31) — see §10.2. target_weeks
     # is still the wanted duration inside that range; recurrence/first_year/last_year are unchanged,
     # so a 'yearly' flexible wish still re-resolves its bounding range every anchor year through the
-    # existing window_for_year()/applies_to_year() helpers below, completely unmodified.
+    # existing window_for_year()/applies_to_year() helpers below.
+    #
+    # SUPERSEDED (weekday-shift fix): "unmodified" held only for the helpers'
+    # signatures, not their behavior. `window_for_year` -> `project_window`
+    # now applies the same per-year weekday snap to BOTH modes (previously
+    # `fixed` only) — a flexible bound keeps its weekday alignment year over
+    # year (a week-shaped bound stays week-shaped) with its span, and so its
+    # search space, held exactly as wide as the anchor year's. See
+    # `app/entities/leave_wish.py::project_window`'s docstring.
     window_mode: str = "fixed"  # 'fixed' | 'flexible'
 
     created_at: datetime | None = None