Add experiment comparison view — side-by-side loss curves and metrics

[SahilKumar75]

Background

ExperimentState (added in app/state/experiment_state.py) now persists all fine-tuning runs in SQLite with full hyperparameters, final loss, perplexity, and loss_history (the full per-step loss array).

Feature request

Build an Experiment Comparison page or panel that lets users:

1. Select two or more completed runs from a table or checklist (toggle_run_selection is already implemented).
2. View overlaid loss curves on a shared axis — one line per run, color-coded.
3. See a side-by-side metrics table showing final loss, perplexity, epochs, LR, LoRA rank, and dataset.

Design notes

• Use app/components/loss_chart.py as the base — it already renders a Recharts LineChart.
• Extend it to accept a list[list[float]] (one series per run) and a matching list[str] for labels.
• The comparison view can live as a new tab on the Experiments page.
• ExperimentState.selected_runs already returns the selected ExperimentRun objects.

Acceptance criteria

• Can select ≥ 2 runs and see their loss curves overlaid on the same chart
• Metrics table updates to show only selected runs
• Works with runs that have different numbers of steps (pad or clip to shorter)
• Empty state shown when fewer than 2 runs are selected