PFX-WB-036: Add import overview table caption semantics

[peshay]

Summary

• add a visually hidden <caption> to the import overview sources table
• add a visually hidden <caption> to the import overview recent import runs table
• add LiveView assertions covering both hidden captions

Scope guard

• only read-only UI accessibility semantics were changed
• no import processing, document handling, persistence, routing, or write-path logic changed

Validation

• mix format ✅
• pre-commit run --all-files ✅
• mix test ⚠️ fails in this environment because PostgreSQL is unavailable (tcp connect (127.0.0.1:5432): connection refused)
• mix test test/portfolixir_web/live/import_overview_live_test.exs --max-failures 1 ⚠️ same PostgreSQL availability failure