Compose: a popup for links — Open / Edit / Delete — and for creating them

Clicking a link in the message shows "Open the link: <URL> | Edit | Delete"
just below it. Open opens http(s)/mailto links in a new tab (a pasted
javascript: link is shown but never run); Edit swaps in an inline URL field;
Delete removes the link and keeps its words. Click elsewhere, Escape or
typing closes it — and Escape stops there instead of also minimising the
compose window.

The toolbar's link button uses the same floating field instead of the
browser's prompt: under the selected text (kept highlighted with a CSS
highlight while the field has focus), at the caret with the address as the
text, or — with the caret inside a link — editing that link rather than
nesting a new one. A bare "example.com" becomes https://example.com.

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

frontend/src/i18n/en.ts

@@ -251,6 +251,10 @@ export const en = {
   "compose.fmtImage": "Insert image",
   "compose.insertSignature": "Insert signature",
   "compose.linkPrompt": "Link URL:",
+  "compose.linkOpen": "Open the link:",
+  "compose.linkEdit": "Edit",
+  "compose.linkDelete": "Delete",
+  "compose.linkSave": "Save",
   "compose.imageTooLarge": "Image is too large (max 5 MB).",
   "compose.attachTooLarge": "File exceeds the 25 MB limit.",
   "compose.attachments": "Attachments",