Task: Land CSV/XML import-export as a qt-profile template capability

Currency is the only entity with CSV/XML import/export anywhere in the codebase — there is no existing generic pattern to templatize against:

• ores.refdata.api/csv/exporter.hpp has exactly one method, export_currency_config; no other entity has a CSV exporter.
• ore::xml::exporter/importer::export_currency_config / import_currency_config are likewise currency-only.
• ImportCurrencyDialog (preview table, checkboxes, progress bar, cancel) is a hand-written, currency-specific QDialog.
• cpp_qt_mdi_window.{hpp,cpp}.mustache have zero hooks for CSV/XML/import/export today.

So this is new capability design, not a "lift existing pattern into mustache" job.

Split into two independently-landable pieces, since they have different generality:

1. CSV/XML domain-layer exporter/importer — per-entity, hand-written (like today), NOT templated. The record shape differs too much entity-to-entity to make a generic serializer worth the codegen complexity for two formats. Codegen's job is only to know an entity has these and wire the Qt calls to them by convention (export_<entity>_config, import_<entity>_config), same as it already does for e.g. generate_fictional_currencies-style generator hooks.
2. Qt wiring — templated. A new opt-in mdi_window flag, has_csv_xml_io: true, gated the same way has_flag_icon gates the flag-image mechanism today. When set, the generated *MdiWindow gets:
   • toolbar actions: Import XML, Export CSV, Export XML
   • exportToCSV()=/=exportToXML()=/=importFromXML() methods calling the entity's csv::exporter=/=xml::exporter=/=xml::importer by the naming convention above
   • a generic ImportEntityDialog<Entity> (header-only template in ores.qt/api, same shape as DynamicComboSetup.hpp's populateDynamicCombo<Entity>) replacing the currency-specific ImportCurrencyDialog, parametrized by:
     • code_of(Entity) / preview column accessors (reuse the mustache field list already used for the entity's list/detail views instead of a second field enumeration)
     • the import call
   • the has_csv_xml_io knob also implies the entity type is copy/move-constructible and has the two free functions above; codegen should fail fast at generation time if either is missing rather than emit code that won't compile.

1. Add ImportEntityDialog<Entity> template to ores.qt/api, modelled on ImportCurrencyDialog but parametrized per the above.
2. Migrate currency's ImportCurrencyDialog call site to it; delete ImportCurrencyDialog.{hpp,cpp} once the migration is verified behaviourally equivalent.
3. Add the has_csv_xml_io schema flag to the qt entity model (Python side) and the corresponding mustache sections in cpp_qt_mdi_window.{hpp,cpp}.mustache.
4. Flip the flag on for currency's model; regenerate; diff against the migrated hand-written version from step 2 — should match modulo naming.
5. Build passes; acceptance met.

• Reusing the full qt.columns list for the import preview table was wrong: fields like version=/=modified_by=/=recorded_at are meaningless before a record is imported. First cut added a derived import_preview_columns (org_loader.py) filtering columns down to non-computed, non-timestamp, non-audit fields — an opt-out exclude-list. PR review caught that this still over-included fields (11 columns for currency vs the 5 the hand-migrated dialog actually shows), and that the --diff verification below should have caught the mismatch and didn't. Replaced with an opt-in :import_preview: true column property instead, so an entity author explicitly marks which columns matter for the import preview — no exclude-list to silently under/over-fire.
• The naive get{{entity_pascal}}s() method name mispluralised ("getCurrencys"). Used the existing entity_pascal_short_plural property (already used elsewhere, e.g. on{{entity_pascal_short_plural}}Loaded) instead of inventing a new per-entity property.
• Verified by running codegen entity generate currency --address ores.cpp.qt --diff and comparing the has_csv_xml_io-gated sections against the hand-migrated code from step 2 — match modulo formatting/comments. Did not adopt the full regenerated file set: currency's Qt layer has substantial pre-existing drift unrelated to this capability (badges, image cache, generator button, NATS notifications, …) that the broader "Sync Qt codegen for currency" story/regenerate plan still needs to reconcile separately.