Forward a message as an attachment

Forwarding quoted the original into a new message, which is the right
thing for passing on something to be read and the wrong thing for passing
on something to be looked at. Quoting rewrites the body, drops the
headers, and re-parents the attachments, so a bounce, a phishing report or
anything else where the message itself is the evidence arrived altered.

Forward as attachment sends the message whole, as a message/rfc822 part.
It costs no upload at all: a message's own blobId is its RFC822 blob and
already lives in the account, so this goes through the same by-reference
path as attach-from-Files and a 40 MB message attaches as fast as a small
one.

It is in the message's own menu, the list's right-click menu, and the
overflow on the reply strip at the foot of a thread, which is the one a
thumb finds on a phone.

Two things fixed on the way, both exposed rather than introduced by this.

The filename rule was subject.replace(/[^\w.-]+/g, "_"), and \w without
the u flag is ASCII: every character of a Russian, Japanese or Chinese
subject failed the class, so those messages downloaded as a row of
underscores. What is actually unsafe in a filename is much shorter than
"not ASCII" -- path separators, the names Windows reserves, the control
range -- so the rule now keeps letters from any script and drops only
those. It lives in one place and the .eml download uses it too.

And the composer's attachment chip set overflow/text-overflow on a span,
where neither does anything, so the name never truncated and the size ran
on after it on the same line. Only long names showed it, which is every
.eml named from a subject.

FEATURES.md

@@ -229,6 +229,19 @@ same query string — so what it builds can be read, edited and learned from.
 - **Attachments** listed with type and size: download, open in a new tab, and an
   inline preview for images and PDFs.
 - **Show original**, **Show headers**, **Download (.eml)** and **Print**.
+- **Forward as attachment** sends the message itself rather than a quotation of
+  it — headers, structure and every attachment intact, which is what a bounce
+  or a phishing report needs and what quoting destroys. It costs **no upload at
+  all**: a message's `blobId` is its own RFC822 blob and already lives in the
+  account, so a 40 MB message attaches by reference as fast as a small one. In
+  the message's ⋮ menu, the list's right-click menu, and the overflow on the
+  reply strip at the foot of a thread, which is the one a thumb finds on a
+  phone.
+- Saved and attached `.eml` files are **named from the subject in whatever
+  script it is written in**. The rule keeps letters and drops only what a
+  filesystem cannot take — path separators, the names Windows reserves, control
+  characters — so a Russian or Japanese subject keeps its own name instead of
+  becoming a row of underscores.
 - **Unsubscribe** where the message carries `List-Unsubscribe`.
 - **Sender details** expand to the full From/To/Cc/Reply-To with addresses.
 - **Message body theming** is off by default — sender HTML is left exactly as it