integrate validators 3-tier warning style

[nautilus69]

Call esqlabsR::validateAllConfigurations() on import, reload, and export
to check xlsx configuration files. Results are mapped into an extended
WarningHandler that now supports both critical errors and warnings.

• Extend WarningHandler with critical_messages, add_critical_error(),
  has_critical_errors(), clear(), and populate_from_validation()
• Update warning modal to show 3 tiers: clean (black bell),
  warnings only (yellow triangle), critical errors (red circle)
• Block export when critical errors exist (button disabled + re-validation
  before writing files)
• Show blocking modal on import when critical errors are found

Closes #193

[nautilus69]

Should be merged after esqLABS/esqlabsR#983 to esqlabs main.

Instructions for Code Review:

Test Case 1: Clean import (no errors)
Test Case 2: Import with critical errors

• Create a ProjectConfiguration.xlsx that points to a Scenarios file missing required columns (e.g., remove IndividualId column)
• Import it
• Expected:
  • Red circle-exclamation icon appears in navbar
  • Blocking modal appears: "X critical error(s) found"
  • Export button is disabled
  • Click the red icon → modal shows critical errors in red, grouped by file

Test Case 3: Import with warnings only

• Create a config where optional sheets (e.g., plotGrids) are missing
• Expected:
  • Yellow triangle icon appears
  • Toast notification about warnings
  • Export button stays enabled
  • Click icon → modal shows warnings in yellow

Test Case 4: Export blocked

• Import a config with critical errors
• Try clicking Export (if somehow enabled)
• Expected: "Export Blocked" modal appears, no files written

Test Case 5: Reload clears and re-validates

• Import a valid config → no critical errors
• Manually break one of the xlsx files (e.g., delete a required column)
• Click "Reload Project" → confirm
• Expected: Warnings/errors from the broken file appear, export disabled if critical

Test Case 6: Reload fixes issues

• Import a broken config → critical errors shown, export disabled
• Fix the xlsx file
• Click "Reload Project" → confirm
• Expected: Previous errors cleared, bell icon resets, export re-enabled