Calendar · Months: a month view, a year view, event chips and cards

CalendarMonthView lays a month in week rows. An event across days lies
over them as a bar in the first free lane, cut at the week's edge;
the rest sit by the hour as chips, and a crowded day keeps the rest
behind "N more". Arrows move a day, Page keys turn the month, and
Enter or a double press opens the day; the cursor holds through the
owner's echo of a turn. Days sit at sevenths of their row, as the bars
do, so text never widens a column.

YearView shows twelve small months; a day with something on it wears a
dot in its first event's hue. EventChip is an event in a line and
EventCard a block with its hours and place. MiniCalendar points to
forms::Calendar. Palette::hue is now the one home for a chart color by
index, for mail labels and events alike.

src/calendar/head.rs

@@ -0,0 +1,66 @@
+use std::rc::Rc;
+
+use gpui::{App, Div, ElementId, FontWeight, ParentElement, Styled, Window, div};
+use jiff::civil::Date;
+
+use crate::{
+    buttons::{Button, ButtonVariant, IconButton},
+    primitives::IconName,
+    theme::{ActiveTheme, ControlSize, TextSize},
+};
+
+pub(crate) type OnDay = Rc<dyn Fn(Date, &mut Window, &mut App)>;
+/// Steps a view back (-1), to today (0) or on (1).
+pub(crate) type Step = Rc<dyn Fn(i32, &mut Window, &mut App)>;
+
+/// A view's title, and beside it Today and the arrows back and on; the arrows drop below a narrow title.
+pub(crate) fn head(
+    id: &ElementId,
+    title: String,
+    back: &str,
+    on: &str,
+    step: Step,
+    cx: &App,
+) -> Div {
+    let theme = cx.theme();
+    let button = |name: &str, icon: IconName, tip: &str, by: i32| {
+        let step = step.clone();
+        IconButton::new((id.clone(), name.to_string()), icon)
+            .variant(ButtonVariant::Ghost)
+            .size(ControlSize::Sm)
+            .tooltip(tip.to_string())
+            .on_click(move |_, window, cx| step(by, window, cx))
+    };
+    let today = {
+        let step = step.clone();
+        Button::new((id.clone(), "today"), "Today")
+            .variant(ButtonVariant::Secondary)
+            .size(ControlSize::Sm)
+            .on_click(move |_, window, cx| step(0, window, cx))
+    };
+    div()
+        .flex()
+        .flex_wrap()
+        .items_center()
+        .justify_between()
+        .gap_2()
+        .child(
+            div()
+                .flex_1()
+                .min_w(theme.label_width())
+                .text_size(theme.text_size(TextSize::Lg))
+                .font_weight(FontWeight::SEMIBOLD)
+                .text_color(theme.colors.fg)
+                .child(title),
+        )
+        .child(
+            div()
+                .flex_none()
+                .flex()
+                .items_center()
+                .gap_1()
+                .child(today)
+                .child(button("back", IconName::ChevronLeft, back, -1))
+                .child(button("on", IconName::ChevronRight, on, 1)),
+        )
+}