refactor: move author-facing mail copy into a Fluent catalogue

Ports carrier's src/i18n.rs and its locales/en-US/main.ftl layout, same
fluent-bundle and unic-langid versions, so the two catalogues stay portable
between them. Only mail the daemon sends an author moves; operator CLI output
stays inline, since the operator runs the binary and reads English.

The error-reply reasons from dispatch and ApplyError are deliberately left
behind: dispatch is pure by contract, so those become reason identifiers
resolved at the mail-building edge in a separate change rather than having the
pure layer reach for a bundle.

Adds a test carrier lacks. FluentResource::try_new and add_resource both panic
on a malformed or duplicate-id catalogue, and nothing touches the bundle until
the daemon first mails someone, so without it a bad catalogue surfaces inside
poll() on a live instance.

Sentences are verbatim; the bodies are now hard-wrapped where they were single
long lines, which reads better in a plain-text client.

daemon/locales/en-US/main.ftl

@@ -0,0 +1,37 @@
+## Loaded once by `src/i18n.rs`; every string the daemon mails to an author
+## lives here, grouped by the mail it belongs to. Operator-facing CLI output is
+## deliberately not here — see the module header.
+##
+## Keep the `-32` style placeables out: these bodies are read in plain-text
+## mail clients and on phones, so they stay short lines and no markup.
+
+## Error replies — a mail the daemon could not apply
+
+reply-subject = twtxt: { $reason }
+reply-body =
+    Your mail could not be applied.
+
+    Reason: { $reason }
+
+    Quoted mail:
+    { $quoted }
+
+## Retention — the one-time warning as a feed approaches its size cap
+
+cap-warning-subject = twtxt: feed at 90% of its size cap
+cap-warning-body =
+    Your feed has reached 90% of the { $cap }-byte cap.
+
+    In hard-limit mode (the default) a full feed rejects new records; delete
+    some with Twt-Delete, or choose FIFO pruning with Twt-Prune: fifo, which
+    drops the oldest records to make room.
+
+## Admission — the mail that hands over the accept token
+
+welcome-subject = twtxt: your feed is ready
+welcome-body =
+    Your smolboid feed is live: { $url }
+
+    Post by sending mail to this address with the twt as the body. Manage it
+    with Twt-Nick, Twt-Delete, Twt-Repost and Twt-Prune frontmatter fields
+    (PLAN.md).