feat(analytics): make dashboard responsive for tablet viewports

[Ummi-001]

Closes #571

Summary

Makes the analytics dashboard usable on tablet-width viewports (768px).

Changes

• frontend/app/analytics/layout.tsx — new shared layout for the analytics route; holds the page shell (background, header) so it's consistent across future analytics sub-pages.
• frontend/app/analytics/page.tsx — dashboard content now uses a responsive CSS grid (single column below lg, two columns at desktop) so charts stack vertically on tablet-width screens instead of overflowing horizontally.
• frontend/components/analytics/DauMauChart.tsx — date-range controls (presets + date pickers) now collapse behind a "Filters" toggle button below the md breakpoint, instead of sitting inline, so they no longer crowd or overlap the chart on narrower viewports.

Acceptance criteria addressed

• No horizontal scroll at 768px width; charts stack vertically.
• Controls remain reachable via the Filters toggle without overlapping chart content.

[vercel]

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