feat: restore project-based sessions and prepare multi-engine providers

[Devin-AXIS]

Summary

• restore projects as the first-level owner of conversations and remove the legacy session-group stack
• add project creation, rename, removal, pinning, activity indicators, and persisted workspace ordering
• introduce a provider-engine registry with the current OpenCode provider/auth/config implementation behind an adapter
• make provider caches and model selection engine-scoped while preserving current OpenCode behavior
• build the shared workspace type subpath for Node runtime imports

User impact

• projects remain stable in the sidebar with their conversations nested underneath
• existing OpenCode model/provider/auth flows keep the same UI and behavior
• future engines can add provider adapters without changing product-level provider flows

Validation

• pnpm --dir apps/app typecheck
• pnpm --dir apps/server typecheck
• focused app tests: 46 passed
• desktop workspace-store tests: 22 passed
• server workspace/session tests: 16 passed
• pnpm --filter @ipollowork/types build && pnpm --dir apps/app build
• live Electron dev startup verified against this worktree
• git diff --check origin/main...HEAD

Known baseline

• node scripts/i18n-audit.mjs --ci still reports the repository's existing orphan/dynamic/plural audit failures on origin/main; this branch removes legacy keys and introduces no dangling t() calls