feat(editor): safely edit existing PDF text

[McanKul]

Summary

• add a bounded source-content classifier and existing-text editing UI
• preserve original files and reject stale, malformed, encrypted, signed, or unsupported content explicitly
• verify previews and saved output with independent qpdf/Poppler checks before publish
• document the supported subset, fixtures, limits, and release behavior
• require PDF engines in CI so coverage cannot silently skip

This completes the v0.4 direct-editing foundation without promising Word-style reflow or universal PDF support. Existing image replacement remains unsupported and is reported honestly.

Verification

• npm run check:versions
• npm run build
• npm test — 60 files, 583 tests passed
• cargo check --manifest-path src-tauri/Cargo.toml
• OFFPDF_REQUIRE_ENGINES=1 cargo test --manifest-path src-tauri/Cargo.toml --lib -- --nocapture — 846 passed, 0 failed, 6 ignored, 0 skipped for missing engines

Closes #11
Closes #12
Closes #33

[McanKul]

Closing this PR so the text-editing work can be reorganized into smaller, reviewable changes. Nothing from this branch has been merged.