fix(postgres): support selectable schemas - #87

Summary

• add a PostgreSQL schema selector that preserves an accessible selection and propagates schema identity through sidebar items, tabs, actions, DDL, table statistics, and active highlighting
• make sidebar refresh publication coherent and latest-wins across connection changes, disconnects, failures, and concurrent schema loads
• require an operation-owned handle for the retained two-phase sidebar API so callers cannot consume another reload or let an older request overwrite a newer one
• bind schema values, qualify identifiers, scope SQL dumps to the selected schema transaction, and preserve PostgreSQL diagnostics including rollback context

Verification

• RED schema-selection run: https://github.com/gridex/gridex/actions/runs/33450320318
• RED schema/action run: https://github.com/gridex/gridex/actions/runs/33452870252
• RED final lifecycle run: https://github.com/gridex/gridex/actions/runs/33458649798
• RED split-pipeline run: https://github.com/gridex/gridex/actions/runs/33460602450
• final GREEN run: https://github.com/gridex/gridex/actions/runs/33461732489
  • 6/6 schema-selection tests
  • 6/6 owned/coherent sidebar publication tests
  • all binding, qualified-DDL, dump, transaction, error-formatting, and schema-highlight filters
  • full suite: 121 executed, 17 environment-gated integration tests skipped, 0 failures
  • swift build passed

The live PostgreSQL integration tests were environment-skipped because no test server was configured on the runner; deterministic adapter-backed PostgreSQL regressions ran successfully.

Closes #80