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/migration.py

@@ -189,6 +189,16 @@ def _migration_12(conn) -> None:
     conn.execute(text("ALTER TABLE frames ADD COLUMN calendar_tasks_cached TEXT"))
 
 
+def _migration_13(conn) -> None:
+    """A day-count-relative start offset for the week view
+    (calendar_week_start_offset), used instead of calendar_week_start's
+    fixed-weekday anchor once the view isn't a literal 7-day week --
+    "start on the most recent Monday" stops meaning much for e.g. a
+    5-day view. Default 0 (starts today) is a behavior-preserving no-op
+    until someone changes the day count away from 7."""
+    conn.execute(text("ALTER TABLE frames ADD COLUMN calendar_week_start_offset INTEGER NOT NULL DEFAULT 0"))
+
+
 MIGRATIONS = [
     (1, _migration_1),
     (2, _migration_2),
@@ -202,6 +212,7 @@ MIGRATIONS = [
     (10, _migration_10),
     (11, _migration_11),
     (12, _migration_12),
+    (13, _migration_13),
 ]