Fix speedtest UX: critique findings

[itsDNNS]

Summary

Addresses all findings from a design critique of the speedtest view:

Bug fixes:

• Add missing .val-bad CSS class -- below-threshold download/upload values were being classified in JS but never styled (invisible to users)
• Fix chart grid lines using hardcoded rgba(255,255,255,0.07) -- invisible in light mode, now uses --border-subtle token
• Fix chart font using hardcoded monospace instead of --font-mono token

UX improvements:

• Add touch event handlers to speedtest chart for mobile tooltip interaction (was mouse-only)
• Replace bare "No speedtest data" text with rich empty state: icon, title, description, and "Run Speedtest" CTA button
• Show elapsed time counter on Run button during speedtest execution ("Running... 15s" instead of just "Running...")
• Fix tooltip viewport overflow with edge detection (flips left when near right edge)
• Cache signal detail fetches so collapsing/re-expanding a row doesn't re-fetch the API

Cleanup:

• Remove decorative hover effect (translateY + purple glow) from chart card -- false affordance on non-interactive element
• Build tooltip content with DOM methods instead of innerHTML
• i18n keys added for new empty state text in all 4 languages (EN/DE/FR/ES)

Test plan

• All 27 speedtest tests pass
• i18n validation passes (all 39 language files in sync)
• Dev instance builds and renders correctly
• Verify chart tooltip works on mobile (touch)
• Verify empty state renders when no speedtest data exists
• Verify light mode chart grid lines are visible
• Verify val-bad class colors below-threshold speeds red