Make "Show original" in the headers dialog the action, not a description

The hint at the foot of Message headers named an action and left you to go
find it. Requested in #236, and the reporter is right that it is the shape of
the thing rather than the size: telling somebody a feature exists is half a
job when the other half is one element away.

Clicking it now closes the headers dialog and opens the original, so it reads
as going deeper rather than as opening a second window.

`tNode` rather than a sentence chopped either side of a button: the sentence
stays whole for whoever translates it, and a language that puts the verb
somewhere else can move the hole rather than being handed two fragments.

The link style needed unscoping to work, which turned out to be a bug of its
own. `.link-btn` was written for the composer's To and Cc labels and scoped to
`.composer-field label`, so the two callers outside it -- the trusted-domain
list in Privacy settings, and now this -- rendered as default button chrome in
the middle of a sentence. The rule is now unscoped and Privacy is fixed by the
same change.

Checked in a browser: the hint reads as a sentence with a dotted-underlined
link in it, clicking swaps one dialog for the other, and the raw message is
there.

Closes #236.

web/src/locales/ja.ts

@@ -216,7 +216,7 @@ export const catalog: Catalog = {
     "Never send one": "送信しない",
     "Not this time": "今回は送信しない",
     "It would go to {address}, which is not where the message came from.": "送信先は {address} で、メールの送信元とは異なります。",
-    "Use “Show original” for the complete raw message.": "完全な原文は「元のメールを表示」でご覧いただけます。",
+    "Use {action} for the complete raw message.": "完全な生メッセージは {action} から。",
 
     // ── Folders, calendar, contacts, files ─────────────────────────────
     "Folder": "フォルダー",