refactor: modularize dashboard into SOLID modules with DI and testability

[sleipi]

Summary

• Extract 1131-line dashboard.ts monolith into 16 focused modules under src/dashboard/
• Introduce dependency injection via DashboardDeps and MaintenanceDeps patterns (consistent with plugin.ts)
• Separate concerns: pure template functions, repo-backed services, route handlers, server wrapper
• Add 67 new unit tests covering all extracted modules (121 total, all passing)

Architecture

src/dashboard/
  services/     - SessionStatsService, DailyTokensService, MaintenanceService (all DI'd via Repos)
  templates/    - Pure render functions (data in -> HTML out), extracted CSS
  routes/       - RouteHandler interface with stats + page routes
  server.ts     - Bun.serve wrapper
  index.ts      - Composition root
src/dashboard.ts - Backward-compatible shim (re-exports for existing tests/symlinks)

Key Changes

• SRP: Each module has exactly one responsibility
• DI: All services depend on Repos interface, not SQLite; MaintenanceService has injectable probability/interval config
• Testability: Templates are pure functions; services/routes fully stubable; maintenance scheduling is deterministic in tests
• Fault tolerance: All error-swallowing behavior preserved and explicitly tested
• Zero behavior change: HTTP contract (/, /api/stats) unchanged; existing e2e tests unaffected