feat(dashboard): create an event from the dashboard - #354

The manager shell resolves the team named in the route and 404s a team the user
does not belong to. A full-screen page has the same need but cannot be a child
of the layout that holds the logic, so the watcher moves into a composable both
can call. No behaviour change.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Adds `buzz.api.events.create_event`, the endpoint behind the dashboard's create
form. Membership decides who may use it: anything below Manager on the team is
refused, as is a non-member.

`Buzz Event` requires a category and a host that the form does not ask for, so
the service fills them in. The host is the team's own, minted on first use since
a new team has none. The category is Meetups, or Zoom Meeting when a Zoom
meeting was asked for — that one is not cosmetic, it is what marks an event as
Zoom-backed.

The Zoom meeting is booked after the event exists and linked back to it, the
same call the Desk button makes. Zoom is checked before the insert so a missing
integration refuses outright rather than half-making an event, and a failure
during the call leaves the event standing rather than rolling it back.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

The contour rings were dense enough to moire at card size, and at one pixel they
broke into a dotted stipple wherever a curve ran near-horizontal. Widens the
spacing and draws the rings two pixels thick, so they read as lines.

The line width becomes a constant rather than a literal repeated across both
gradient stops — the test that pins ring spacing was matching on it by accident.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

One control for both kinds of location: the team's venues, searched as you type,
and a Zoom meeting under Virtual. A name nothing answers to offers to add a
venue, carrying what was typed into the dialog.

Venues read and write through `frappe.client`; Event Venue's team query
condition and permission hook are what scope them, so the team filter is only
narrowing what the picker asks for.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Start and end, each a date beside a time, and a time zone picker searchable by
city, country, everyday name or zone id.

The zone data all comes from Intl, so the list tracks the browser's own tzdata
with nothing to maintain. Country needs inverting: there is no zone-to-country
lookup, only region-to-zones, so every two-letter code is swept once, on first
open rather than on mount. Zones IANA has renamed still arrive under the old
name, and a picker offering "Calcutta" to an organiser in India is wrong, so the
ones a reader would notice are mapped forward.

Dates are constrained rather than validated after the fact: the start cannot be
in the past, the end cannot precede the start, and an end that has fallen behind
follows the start rather than a multi-day span collapsing to one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Reached from the Create Event button on the team pages, at
/manage/<team>/events/new. Full screen rather than inside the manager shell, so
the team comes from the route guard rather than the sidebar.

The page is the event as it will look: a banner, the title as the headline, and
the rest arranged around them. Without an image the banner draws the same
generated pattern the event cards use, seeded by the title as it is typed —
debounced, because a gradient cannot be transitioned, and blurred across the
change, because that is the only way to hide one.

An event needs a name, when it runs and where before it can be created; the
description is optional. Uploads are restricted to raster images, both in the
picker and on the file that comes back, and a rejected file now says so.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

The create page only checked team membership, so a Viewer reached the full
form and lost the work to a 403 on save. The server is the authority and
already refuses anything below Manager; this just stops the form pretending
otherwise.

An alert sits under the header and every control is disabled. The pickers
take frappe-ui's own disabled prop, so the greying comes from the design
system. Three controls need more than that: the timezone combobox supplies
its own trigger, which does not inherit the disabled state; the editor is
contenteditable, so it takes editable instead; and the banner's hit area is
a div, which no disabled prop reaches.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Buzz Event already refuses this on save, but the form gave no sign until
the round trip came back. Two layers now, because neither covers the other.

The end time picker takes a min of the start time, which bounds the list it
offers and reverts a typed value below it. That only applies to a single-day
event: a span running into another day may well end earlier in the day than
it began, so the bound lifts once the end date differs.

The picker's min cannot cover everything, so the check stays alongside it.
Min is inclusive, which still allows an end landing exactly on the start,
and it only constrains new input — an end time picked while the event was
multi-day is left untouched when the end date is pulled back onto the start
date. Both show the same message under the End row and hold the save.

Also renders the server's own validation errors, which arrive as `messages`
rather than a single `message` and so came through blank.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>