Update from code changes: sync xchat translations to Chat XDK v0.4.0

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Summary

Syncs the Spanish, Japanese, Portuguese, and Korean X Chat pages to the current English Chat XDK v0.4.0 source. The upstream English work rewrote every xchat page around session-scoped identity, SDK-generated message ids, opt-in signing-key and conversation-key session stores, and event-based replies and reactions with a reply_preview_validation outcome. This PR carries the same structural rewrite and code snippets across all six language tabs into each localized page and keeps the translations current as further English follow-ups land. Triggered by #268.

Changes

• es/xchat/, ja/xchat/, pt/xchat/, ko/xchat/ — Seven pages per locale (cryptography-primer, getting-started, groups, media, real-time-events, troubleshooting, xchat-xdk) match the English v0.4.0 structure and code.
• Getting started tutorials rewritten around set_identity and the new prepare/send flow, with SDK-generated message_id and version-aware import_keys.
• Chat XDK reference pages document the two opt-in session stores (set_signing_keys, set_cache_keys), the short decrypt_event(s) and encrypt_message call forms, event-based encrypt_reply and encrypt_add_reaction, and the reply_preview_validation semantics.
• Groups, media, and real-time events pages updated for identity-free prepare calls and session-store-based live decrypt.
• Troubleshooting pages carry entries for missing session identity and Invalid reply-preview outcomes.
• Cryptography primer picks up the reply-preview signature paragraph in the signatures section.
• Groups pages add a "Key changes from departed members" section explaining why verified decryption fails when a signer leaves and when to fall back to extract_conversation_keys.
• Troubleshooting pages add a "Your local public key never matches the account's registered keys" section with a six-language byte-suffix comparison, guidance on numeric key-version comparison, and a cross-reference in the signature-failure causes list.

Context

The upstream English rewrite explicitly deferred the localized mirrors to translation; this PR closes that gap so the translated docs no longer describe removed APIs (per-call sender_id / signing_key_version, caller-minted UUID message ids, set_key_version), and keeps the mirrors in sync as follow-up English guidance lands.

Update history

• Initial run: synced es/, ja/, and pt/ xchat pages to Chat XDK v0.4.0 (7 pages per locale).
• Follow-up: added Korean (ko/) xchat translations for the same seven pages; tightened Spanish and Portuguese SEO descriptions; quoted two YAML descriptions containing colons that broke frontmatter parsing.
• Follow-up: mirrored the English "Key changes from departed members" (groups) and "Your local public key never matches the account's registered keys" (troubleshooting) additions into all four locales, plus the new signature-failure bullet cross-linking to the groups section.

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Update from code changes updated this PR.

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