Support string and GUID-based package UIDs

[mehdii-me]

LoadZa now treats item UIDs as opaque string identifiers, supporting GUIDs and other external package IDs while preserving compatibility with numeric UIDs in existing job files.

Changes

• Changed Item.uid, Placement.item_uid, and Unplaced.item_uid to strings.
• Preserve explicit string UIDs during deserialization and serialization; normalize numeric UIDs to strings.
• Keep generated UIDs sequential ("0", "1", …), independent of explicit UIDs.
• Retain duplicate UID validation.
• Replace UID-based solver ordering with stable input-order tie-breaking.
• Store UIDs as text in SQLite and migrate existing integer-affinity UID columns.
• Update frontend types and document the job JSON UID format.

Validation

• Full test suite: 144 passed, 4 skipped
• Ruff: passed
• Web type-check and production build: passed
• Verified GUIDs survive deserialization, solving, storage, and plan JSON serialization.