Calendar · Time grid: week and day views, drag to make and to resize

TimeGrid sets days side by side under a column of hours. Each event is
a card by its hours, clipped to each day it touches, and events that
overlap share the width in lanes. A drag on empty time makes an event
by quarter hours through forms::pen; a drag on an event's lower edge,
on the day it ends, moves its end, each drag checked against its
owner. The grid opens at half past seven, or an hour and a half
before now when today is shown and that is later.

CurrentTimeIndicator marks the minute now on today and moves with the
clock. TimezoneOverlay adds another zone's hours beside the grid's.
AllDayRow lays whole days across their columns in lanes.
CalendarWeekView and CalendarDayView are one view of seven days or
one: a header that steps and comes back to today, the days' names
stacked so they fit narrow columns, the whole days, then the hours.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

AGENTS.md

@@ -91,7 +91,7 @@ Ely GPUI Component. A component library for GPUI, in light and dark.
 - A rounded box does not clip its children. Each layer inside takes the radius itself; `checker` takes one.
 - A window keeps one `feedback::Toaster` and renders one `ToastViewport`; anything holding the entity pushes to it. Messages share `primitives::Severity`.
 - Only gpui can build `ExternalPaths` with paths, so drop rules live in `forms::files::dropped`, where tests reach them. `taken` adds file kinds by extension, for drops and the dialog alike (`DropZone::kinds`).
-- A stroke drawn with the pointer goes through `forms::pen`: a press starts it, a drag extends it, and the lift hands it over, in or out of the pad (`SignaturePad`, `generative::InpaintCanvas`).
+- A stroke drawn with the pointer goes through `forms::pen`: a press starts it, a drag extends it, and the lift hands it over, in or out of the pad (`SignaturePad`, `generative::InpaintCanvas`, `calendar::TimeGrid`'s drag on empty time).
 - gpui runs key bindings before key listeners. A container takes a child's bound key through the action with `capture_action`, as `Form` takes `Submit`.
 - `ScrollHandle::scroll_to_item` runs in the container's prepaint against last frame's bounds and overflow, so a call before a list's first frame does nothing. Call it from a child's prepaint once the handle has bounds, then request one frame (`navigation::editor_tabs`). A floating list reveals its cursor, and a strip its focused item, through `forms::options::reveal` and `revealer`. A focused Tab stop deeper in a scroll box sets the box's offset whole from its own painted bounds in prepaint, once per focus, through `layout::bring_into_view` along the box's axis; the canvas measures the stop with its ring (`messaging::GifPicker`). Never poll layout with `request_animation_frame`.
 - gpui animations run on the wall clock; timers run on the executor's clock, which tests advance by hand. A test that needs an animation settled turns on the theme's reduced motion. Clocks read `background_executor().now()` and wake only their own view.