Copy an incident to the clipboard as Markdown

A button in the incident header (also `y`, and "Copy incident" in the
more menu) puts everything the page knows on the clipboard, for pasting
into a chat or an agent prompt with no integration involved.

The text carries the facts, every alert with all its labels and
annotations (the page only shows summary or description), the timeline
with notes in full, and the "Seen before" resolution notes. Times are
ISO 8601 and users are named rather than "you", since relative and
first-person wording is ambiguous once pasted elsewhere.

The async clipboard API needs a secure context and this server is often
reached over plain HTTP, so it falls back to execCommand.

Web UI only: no endpoint or JSON shape changed, so nothing to mirror in
terdut-tui.

internal/web/static/js/ui.js

@@ -48,6 +48,7 @@ const ICONS = {
   note: ['M5 4h14v12l-4 4H5z', 'M15 20v-4h4', 'M9 9h6M9 13h4'],
   archive: ['M3.5 5h17v4h-17z', 'M5 9v10h14V9', 'M10 13h4'],
   flag: ['M5 21V4', 'M5 4h11l-2 4 2 4H5'],
+  copy: ['rect:9,9,11,11,2', 'M5 15V6a2 2 0 0 1 2-2h9'],
   trash: ['M4 7h16', 'M9 7V4h6v3', 'M6 7l1 13h10l1-13'],
   chevronLeft: ['M15 18l-6-6 6-6'],
   chevronRight: ['M9 6l6 6-6 6'],