feat(spec-site): replace all confirm()/alert() with custom modal

[AngryJay91]

Closes #13

Changes

Replaced all native confirm()/alert() calls with custom modal via useConfirm composable across 11 page files:

• DashboardPage.vue — convertToEpic, convertToStory
• DocsEditor.vue — save validation/error
• DocsPage.vue — restoreRev
• MemosPage.vue — createDocFromMemo, deleteReply, convertToStory, convertToInitiative, createMemo
• MockupEditorPage.vue — onDelete
• MockupListPage.vue — deleteMockup
• MockupViewerPage.vue — copySpec
• board/BoardAdmin.vue — handleDeleteTask
• board/StoryDetailPanel.vue — handleMergeOk
• retro/RetroHeader.vue — nextPhase, handleReset
• retro/RetroPage.vue — handleExport

SlashCommand.ts was already migrated (confirmed clean).

Pattern used

const { showConfirm, showAlert } = useConfirm()
// confirm → await showConfirm(...)
// alert   → await showAlert(...)

All functions made async where needed.