feat(app): make new tasks project-first

[jhe009809-hue]

Summary

• Make projects the primary workspace model by removing the legacy ungrouped/default-workspace presentation from the sidebar.
• Add exclusive project/task selection: selecting a task clears the project highlight, while selecting a project clears the active task.
• Route new tasks to the current or most recently used regular project.
• Let users start from the empty launcher without creating data upfront. The first submitted message now creates an untitled project and its first task automatically.
• Default first-project creation to OpenCode without showing an engine-selection dialog. Engine selection remains available when users explicitly create a new project.
• Update the project creation dialog with the new OpenCode and DeepSeek Harness engine cards.
• Move the active engine indicator into the composer and add the selected project context to the conversation header.
• Refine composer controls, empty-submit feedback, placeholder styling, tab motion, dark-mode states, and related localized copy.
• Add focused coverage for project selection, first-project creation, composer behavior, and startup routing.

Validation

• pnpm --filter @ipollowork/app typecheck
• 54 focused tests passed across:
  • sidebar projects
  • startup session routing
  • new-conversation tabs
  • composer queue behavior
  • composer model/mode controls
  • composer plus menu
• Maintainability audit and git diff --check passed.

Scope note

Starter-page spacing and responsive-layout adjustments are intentionally excluded from this PR and remain as uncommitted local changes.