Post-audit cleanup batch: dead email forks, Docker extension drift, English error locale (#653)

* chore(email): remove dead, diverged email-template forks (audit E2)

extensions/general/email/lib/{invoice,reminder}-templates.ts had zero importers and had diverged from the live lib/email/* copies (which carry later i18n / CSP / Räntelagen-dunning fixes). Pure deletion of a drift hazard.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* fix(docker): hosted preset was missing skatteverket / invoice-inbox / document-extraction / cloud-backup (audit E7)

docker/extensions.hosted.json shipped only 5 of the 9 extensions in extensions.config.json — so a Docker 'hosted' image silently ran without Skatteverket filing, the invoice inbox, document extraction and cloud backup. Aligned with the hosted config.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* fix(errors): return English error messages on the en locale (audit C9)

The structured-error branches in getErrorMessage returned hardcoded Swedish regardless of locale, so English users saw Swedish prose. For the en locale, prefer the registry's English message for any known code; the Swedish (default) path is left entirely unchanged, and codes absent from the registry still fall through. + regression test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docker/extensions.hosted.json

@@ -1 +1 @@
-{"extensions": ["enable-banking", "email", "arcim-migration", "tic", "mcp-server"]}
+{"extensions": ["enable-banking", "email", "arcim-migration", "tic", "mcp-server", "cloud-backup", "skatteverket", "invoice-inbox", "document-extraction"]}