refactor: split DashboardHome.tsx into focused components

Before: 489 lines, 5 concerns in one file
After: 4 files, each with one concern

Extracted:
- RepoDetailView.tsx (140 lines): single-repo view with tabs, header,
  back navigation. Receives repo data + callbacks as props.
- RepoListView.tsx (81 lines): repo grid with header, add/import
  buttons, stats. Pure presentational.
- lib/api-errors.ts (32 lines): extractErrorMessage, isUpgradeError,
  safeStringify. Generic utilities, not dashboard-specific.

DashboardHome.tsx (222 lines): now just state management, API handlers
(addAndIndex, handleGitHubImport, handleReindex), and modal orchestration.

Also:
- Deduplicated addRepo and githubImport into shared addAndIndex helper
- Moved AddRepoForm outside AnimatePresence (was incorrectly nested)
- All modals rendered at the bottom of DashboardHome, not inside views

Zero visual/behavioral changes. Build passes. 13 tests green.

Closes OPE-16

Author: DevanshuNEU