NO-2080: Add typed signature input mode

[vivekjyani]

1) Spec Context

This PR implements NO-2080 by adding typed-signature support alongside draw mode, and by tightening signature behavior for edit/reopen flows across standard, table, and collection signature fields.

2) What Changed (Code + Files)

Core implementation

• Sources/JoyfillUI/View/Fields/SignatureView.swift
• Sources/JoyfillUI/View/Fields/TableView/TableSignatureView.swift

Key updates

• Added draw/type toggle support in signature input.
• Added typed-signature rendering flow (TypeToSign + typed snapshot generation).
• Added mode-specific save behavior:
  • Draw mode preserves existing canvas snapshot behavior.
  • Type mode trims input, treats empty value as clear, snapshots typed text, and clears stale draw line data.
• Restored previously typed signature text when reopening an existing typed signature for edit.
• Updated typed-signature font styling and removed opacity-based text styling.
• Updated table signature entry behavior to reset mode/editability before presenting the sheet.

3) UI Test Coverage Added

Files changed

• JoyfillSwiftUIExample/JoyfillUITests/SignatureField/SignatureFieldUITestCases.swift
• JoyfillSwiftUIExample/JoyfillUITests/TableField/TableNumber_Block_DateFieldTest.swift
• JoyfillSwiftUIExample/JoyfillUITests/Collection/CollectionFieldTests.swift

New test scenarios

• Existing saved signature opens read-only preview first (with Edit action).
• Signature mode resets to Draw when reopening a fresh signature sheet.
• Previously typed text is restored on edit.
• Typed signatures are row-scoped in table/collection flows (no carry-over to new rows).

4) Public API / Docs

• No public API changes.
• No docs update required.

5) Reviewer Notes

• Primary focus areas:
  • mode-specific save logic and state cleanup
  • typed-text restore path on edit
  • row-scoped behavior in table/collection signature flows