feat: comprehensive e2e tests for dashboard

[sleipi]

Summary

• Expand e2e test coverage from 2 to 42 tests across 11 new behavior-focused spec files
• Enrich seed data with multiple sessions, models (gpt-5.3-codex, claude-sonnet-4), plan/build modes, subagent child sessions, and multi-day usage history
• Add separate empty-DB seed script for empty state testing

Test Coverage

Spec File	Tests	Feature
it_renders_stats_bar	3	Stats bar with Today/Week/Month/Last Month
it_renders_daily_chart	5	60-day bar chart, SVG polyline, legend
it_renders_model_chart	4	Stacked model chart with legend
it_renders_session_card_details	6	Title, ID, tokens, mode rows, timestamp
it_applies_recency_styling	2	Active (<5min) vs no-class (>24h)
it_filters_by_directory	5	Dropdown, filtering, ?dir= query param
it_returns_directories_api	3	JSON array, seeded dirs, no duplicates
it_aggregates_subagents	3	No own card, agent badge, token aggregation
it_shows_empty_state	2	Empty message, stats bar with zero values
it_auto_refreshes	3	Refresh badge, periodic fetch, timing display
it_renders_two_column_layout	4	Two-col container, left/right panel content