feat(decisions): replace browser confirm() with custom delete confirmation modal

[neethika12]

Summary

Replaces the browser's native confirm() dialog on decision delete with a polished inline modal that matches the existing workspace delete design system.

• decision-list.component.ts: Removed deleteDecision(), added requestDeleteDecision(), cancelDeleteDecision(), confirmDeleteDecision() with proper loading/error state
• decision-list.component.html: Delete button now triggers modal; modal includes trash icon, decision title, warning copy, Cancel + confirm buttons, inline error fallback, backdrop click to dismiss
• decision-list.component.css: Modal backdrop + card styles with fade/slide animations matching dashboard design
• decision-list.component.spec.ts: 6 new test cases covering modal open, cancel, backdrop dismiss, confirm flow, and double-click guard

Behaviour

• Clicking Delete opens the modal (no page freeze)
• Clicking Cancel or the backdrop closes it
• Clicking Yes, Delete calls the API, shows Deleting… state, closes on success
• If the API fails, an inline error appears inside the modal without closing it

Closes Sentinent-AI/Sentinent#29