[Bug](supernova): supernova displays default silence duration of 2 without units

[ArtieReus]

Problem 1: Default silence duration shown without units

In the Create Silence form, the default duration is displayed as 2, but no time unit is shown (e.g., 2h).
This is misleading for users and inconsistent with expected UI behavior.

Location:
apps/supernova/src/components/silences/CreateSilence.tsx
https://github.com/cloudoperators/juno/blob/main/apps/supernova/src/components/silences/CreateSilence.tsx#L64

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Problem 2: Default dropdown value is hard-coded instead of using the shared defaults list

The default value for the silence duration dropdown is manually set in the component instead of being sourced from the shared list of default values.

This causes inconsistency between CreateSilence.tsx and the centralized defaults in silenceHelpers.ts.

Expected:
Use the default values defined in:
apps/supernova/src/components/silences/silenceHelpers.ts

juno/apps/supernova/src/components/silences/silenceHelpers.ts

Line 6 in 33a7810

export const DEFAULT_DURATION_OPTIONS = [

Expected Behavior

Display units

Screenshots