[Dashboard]: Add Graphs tab and vanilla SVG curve rendering to Dashboard session view

[AaronProbha18]

Summary

Update the dashboard UI to include a tabbed interface on the session page and render Loss, Metric, and Learning Rate curves using native SVG elements without adding external dependencies.

Scope

• Add tab navigation (Logs / Graphs) and chart containers to fenn/dashboard/templates/session.html
• Add styling for tab controls, active states, and chart cards in fenn/dashboard/static/style.css using existing CSS variables
• Implement tab switching and a renderLineChart helper in fenn/dashboard/static/app.js
• Hook chart rendering into the existing refresh() polling cycle so graphs render once the session status transitions away from running

Acceptance Criteria

• Users can seamlessly switch between "Logs" and "Graphs" panels on the session page
• An empty/placeholder state displays while the session status is running
• Loss, Metric, and Learning Rate curves render cleanly as SVG polylines upon session completion
• Visuals strictly use existing CSS custom properties (--accent, --bg-surface, etc.) without introducing build steps or third-party libraries

Parent Issue

#290

[AaronProbha18]

hey @ApusBerliozi
just noticed this is missing the Feature type. The rest of the sub-issues have it, so I just wanted to give you a heads-up :)