Week view: relative start-day offset for non-7-day counts; hide week-only settings elsewhere

calendar_week_start's fixed-weekday anchor ("start on the most recent
Monday") stops making sense once the view isn't a literal calendar
week, so a non-7-day week view now starts calendar_week_start_offset
days from today instead (0 = starts today, negative/positive = past/
future) -- calendar_week_start still governs at the default 7 days,
unchanged.

Also hides the Calendar tab's week-only fields (days to show, layout,
start offset) unless View is actually set to Week, and further hides
the new start-offset field specifically when Days to show is 7 (where
it has no effect). "Week starts on" stays visible for Month too, since
it actually still applies there.

server/app/models.py

@@ -207,6 +207,15 @@ class Frame(Base):
     # columns or stacked bands (see calendar_render.py's _build_week).
     calendar_week_days: Mapped[int] = mapped_column(Integer, default=7)
     calendar_week_layout: Mapped[str] = mapped_column(String, default="horizontal")  # "horizontal" | "vertical"
+    # Only used when calendar_week_days != 7 -- calendar_week_start's
+    # fixed-weekday anchor ("start on the most recent Monday") stops
+    # making sense once the view isn't a literal calendar week, so a
+    # non-7-day view instead starts this many days from today (0 =
+    # starts today, negative = starts in the past, positive = starts in
+    # the future). Ignored (calendar_week_start governs instead) at the
+    # default 7 days, so this has no effect until someone actually
+    # changes the day count.
+    calendar_week_start_offset: Mapped[int] = mapped_column(Integer, default=0)
 
     # Optional task list, week view only -- takes the space of one day
     # slot rather than adding an extra one (see calendar_render.py's