feat(frontend): composite primary key support end-to-end (#148)

[saurabh500]

Summary

Implements frontend composite primary key support across the stack, resolving #148.

Builds on the SQL Server backend composite PK work in #146/#147. Existing single-PK behavior is preserved; composite-PK tables now display correctly and inline edits route through dedicated composite commands.

Changes

Backend (Rust)

• models::TableColumn — added pk_ordinal: Option<u32> (1-based position within the PK, matching SQL Server key_ordinal, MySQL SEQ_IN_INDEX, SQLite pragma pk, Postgres kcu.ordinal_position).
• All four drivers (sqlserver, mysql, postgres, sqlite) now populate pk_ordinal in get_columns and get_all_columns_batch. Postgres reads information_schema.key_column_usage.ordinal_position. View columns are set to None.
• driver_trait — new default methods update_record_composite and delete_record_composite. When pk_cols.len() == 1 they delegate to the single-PK path; otherwise they return a descriptive error so non-overriding drivers fail loudly instead of corrupting data.
• Tauri commands — added update_record_composite / delete_record_composite, registered in lib.rs.

Frontend (TypeScript / React)

• TableColumn type — added pk_ordinal?: number in both src/types/editor.ts and src/types/schema.ts.
• DataGrid — new optional pkColumns?: string[] prop alongside legacy pkColumn. Builds pkIndexes memo, derives isCompositePk, and routes inline cell updates through update_record_composite when composite. Single-PK path unchanged.
• Editor.tsx — derives the ordered composite PK list from columnMetadata (filter is_pk, sort ascending by pk_ordinal) and passes it to DataGrid.
• SchemaDiagram — groups foreign keys by constraint name and styles composite-FK edges distinctly (thicker purple line + constraint label).

Verification

• cargo check --lib ✅
• cargo test --lib ✅ 605 passing (2 pre-existing failures in updater::tests unrelated to this PR)
• npm run typecheck ✅
• Lint on touched files clean (one pre-existing unrelated error in SchemaDiagram useEffect)

Notes

• Read-only first. SQL Server remains read-only Phase 1; the default trait error path is sufficient. MySQL / Postgres / SQLite still hit the single-PK delegation when ordinal indicates a true composite, so callers that previously silently used only the first PK column now receive an explicit error instead of corrupting data. A follow-up will add per-driver composite write implementations as needed.
• Existing single-PK tables: zero behavior change.
• pk_ordinal is intentionally optional in the TS type so any cached column metadata from older sessions still deserializes.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com