feat(desktop): add markdown format option to session export

[harrykamboj1]

Summary

Session History's Download action only produced JSON, which is useful for re-import but not for reading or sharing a transcript. This adds a format picker (JSON / Markdown) to that button and wires Markdown through to the same formatter the CLI already uses for goose session export --format markdown, so both paths produce identical output.

The formatter lived in goose-cli, which the ACP server (in the goose crate) cannot depend on without a dependency cycle, so it moves to goose/src/session/export_markdown.rs and both crates now share it. The move is a pure relocation — git records it as a 92% rename and the 33 unit tests move unchanged.

Changes:

• Shared formatter: goose-cli/src/session/export.rs → goose/src/session/export_markdown.rs, alongside export_session_to_markdown from goose-cli/src/commands/session.rs. No Cargo.toml edits.
• ACP: ExportSessionRequest gains a format field (SessionExportFormat, #[serde(default)], defaults to Json), so clients omitting it still receive JSON. Regenerated acp-schema.json and ui/sdk/src/generated/.
• New SessionManager::export_session_markdown. export_session and the SessionStorage trait are untouched, so Nostr sharing and diagnostics are unaffected.
• Desktop: the Download button becomes a dropdown, setting the right MIME type and file extension per format.
• i18n: three new keys added to en.json and all 15 translated catalogs.

Notes for reviewers:

• Markdown uses the user-visible projection (Conversation::user_visible_messages()), matching CLI output byte-for-byte and excluding assistant-only content.
• Markdown is a read-only artifact by design — Import Session accepts JSON formats only, which the menu wording reflects.
• Empty sessions export a "no messages" placeholder over ACP rather than erroring. CLI behavior is unchanged.
• The 15 locale translations are derived from existing export wording in each catalog (e.g. sessions.toast.exported, sessions.action.export) so punctuation stays language-correct. A native-speaker glance at ko and hi would be welcome.

Testing

Automated:

• cargo test -p goose --lib session::export_markdown — 33 relocated tests plus a new markdown_export_handles_empty_conversation
• cargo test -p goose-sdk-types --lib custom_requests — asserts a request without format defaults to JSON, and that markdown deserializes
• cargo test -p goose-cli --lib, cargo fmt --check, cargo clippy --workspace --all-targets -- -D warnings
• just check-acp-schema — generated files in sync with the Rust types
• pnpm run lint:check (typecheck + eslint + i18n-check + locale validation across all 15 locales) and pnpm run test:run — 592 passing

Manual:

• Exported the same session as Markdown from the UI and via goose session export --format markdown, then diffed the two files
• Verified JSON export is unchanged, "View JSON" in the in-session menu still works (proves the default reached the old call site), and an empty session exports the placeholder without an error toast

Related Issues

Fixes #10253
Discussion: N/A

Screenshots/Demos (for UX changes)

[The-Best-Codes]

Merge conflicts!