Fix invalid snap refresh flag and prevent snap/flatpak errors from aborting update script

[Copilot]

snap refresh --stable is not a valid command and causes an immediate script exit due to set -e, halting all subsequent update steps including flatpak.

Changes

• checkforupdates.sh
  • Replace snap refresh --stable with snap refresh (no such --stable flag exists)
  • Add || echo "Warning: ..." fallback to both snap refresh and flatpak upgrade so non-fatal failures emit a warning and allow the script to continue

# Before
snap refresh --stable

# After
snap refresh || echo "Warning: snap refresh encountered an error, continuing..."
flatpak upgrade --assumeyes || echo "Warning: flatpak upgrade encountered an error, continuing..."

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