feat: route accounts through sidecar sqlite store

[linhay]

Summary

• add sidecar-owned SQLite account store and unified management APIs
• route GetTokens account CRUD through sidecar unified account APIs
• record execution plan and migration/delete-legacy checklist in docs-linhay space

Tests

• sidecar: go test ./internal/...
• GetTokens: go test ./internal/...
• frontend: npm run typecheck
• frontend: npm run test:unit (614 passed)
• frontend: npm run build (passes with existing Vite chunk size warning)

Remaining rollout checks

• real desktop validation for account list/create/edit/disable/priority/delete/restart
• real profile migration dry-run, commit, credential update validation, then delete legacy sources

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