Show the language model's opinion on a message

When inbuxa-server's AI spam classification is on, it records the model's
answer in an X-Spam-LLM header: a tag (LLM_<category>[_<confidence>]) and,
in parentheses, the model's explanation. The full message now asks for it,
and where it's there:

- the message details show "Language model's opinion" beside the spam
  filter's own working, with category, confidence and explanation;
- a message in Junk carries a banner saying the same.

Both say it's one of several signals the spam filter weighed, never the
reason on its own, as the server's spec requires. The explanation is model
output and is only ever rendered as text. Nothing shows without the header,
so a server without the feature, or with it off, looks as before.

Translations: two new strings, "Language model's opinion" and "One of
several signals the spam filter weighed", in all eight catalogues (16
entries). Category and confidence come from the server and aren't
translated.

web/src/jmap/types.ts

@@ -268,6 +268,8 @@ export interface Email {
   "header:Received:asText:all"?: string[] | null;
   "header:X-Spam-Status:asText"?: string | null;
   "header:X-Spam-Result:asText"?: string | null;
+  /** inbuxa: the language model's opinion, when AI spam classification is on (lib/llmOpinion). */
+  "header:X-Spam-LLM:asText"?: string | null;
 }
 
 export interface Thread {