feat: list saved PGN positions for active chapter

[levallem]

Closes #24

Summary

Implements CMS-024I by completing the first visible read path for persisted PGN positions.

Saved PGN position snapshots for the active chapter are recovered into a ProjectTab-owned cache and displayed read-only, separately from selected puzzles.

Motivation

CMS-024H completed the write path from PGN review to chapter-scoped persistence. CMS-024I makes that persisted material recoverable and visible after reopening a project, without introducing board loading, editing, deletion, export, or generic editorial abstractions.

Changes

• Add a dedicated PGN positions cache in ProjectTab, contextualized by project path + chapter ID.
• Refresh on project/chapter lifecycle changes and after a CMS-024H snapshot add.
• Refresh Inserted and AlreadyExists from the SQLite source of truth.
• Replace loaded data with a visible fail-closed state on read failure.
• Add a separate read-only Saved PGN positions section with White, Black, Event, ply, and selected FEN.
• Keep PGN positions separate from selected puzzles, target counts, reviews, and exports.
• Add all six translations and document the persisted PGN workflow.

Validation

• cargo fmt --check
• cargo clippy --locked --all-targets -- -D warnings
• cargo test --locked — 584 tests passed, 0 ignored
• cargo check --locked --all-targets
• cargo build --locked
• git diff --check

Screenshots

Not required for this task; behavior is covered by ProjectTab state/integration tests.

Database / migration impact

None. CMS-024I reuses project schema v4 and list_pgn_position_snapshots_for_chapter(...); PROJECT_SCHEMA_VERSION remains 4.

Scope

Exactly 8 files: README.md, src/project_tab.rs, and six translation bundles.

No changes to src/main.rs, PGN review/import code, project persistence, schema/migrations, exports, puzzle persistence, engine, search, favorites, or dependencies.