Save flow metadata fix

[diiviikk5]

Summary

Refers to #38
This PR fixes local persistence reliability in the Ionic React app by making save flows async-safe and null-safe.

Changes

• Govt-Billing-React/src/components/Menu/Menu.tsx
  • Made doSave async and awaited _getFile(...).
  • Preserved created timestamp and updated modified.
  • Added safe fallback toast on missing metadata/save failure.
  • Awaited _saveFile(...) in doSaveAs.
• Govt-Billing-React/src/components/NewFile/NewFile.tsx
  • Made newFile async and awaited _getFile(...).
  • Added guard alerts for missing metadata/save failure.
• Govt-Billing-React/src/components/Storage/LocalStorage.ts
  • Made _getFile null-safe for missing/invalid JSON.
  • Updated _getAllFiles to skip invalid entries.

Why

Previously, _getFile(...) promises were used like resolved objects in save flows, which could break metadata handling and lead to inconsistent file persistence.

Flow Diagram

flowchart TD
  subgraph Before
    A1[Save/New File] --> B1[_getFile returns Promise]
    B1 --> C1[Read data.created immediately]
    C1 --> D1[Invalid metadata may be written]
  end

  subgraph After
    A2[Save/New File] --> B2[await _getFile]
    B2 --> C2{Data valid?}
    C2 -- Yes --> D2[Preserve created, update modified]
    D2 --> E2[await _saveFile]
    C2 -- No --> F2[Show alert/toast and stop safely]
  end

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