feat(glook-8): replace timeline line chart with date-anchored bar chart

[maescomua]

Summary

• Replaces SVG line+area chart in TimelineChart (dev detail page) with <rect> bars positioned by actual date on a fixed [90 days ago, today] X-axis
• Bars always end at "This week" — inactive developers now show empty right space instead of a truncated axis
• X-axis labels are fixed: cutoff date / midpoint / "This week" (no longer data-density-dependent)

Changes

• src/app/report/[id]/dev/[login]/page.tsx — TimelineChart function only; all 4 chart instances (commits, lines, complexity, AI %) fixed uniformly
• Gaps in data are structural — missing weeks have no bar, connecting line segments are gone by design
• Bar height floored at 1px (Math.max(..., 1)) so zero-value weeks render as a hairline rather than disappearing
• Empty state guard lowered from < 2 to < 1 (single bar is valid for a bar chart)
• Org report page has a separate TimelineChart copy with the same issue — explicitly out of scope (no ticket yet)

Test Plan

• Start mock dev server: npm run dev:mock
• Open any report → click a developer
• Verify four bar charts appear in "Activity Over Time"
• Verify right-edge label reads "This week" on all four charts
• Verify left-edge label shows a date ~90 days ago
• Verify bars span the full date range (not bunched at the left)
• Hover a bar — tooltip shows week date and value, vertical guide line appears
• Run tests: npm test (648 passing, all in src/lib)

Closes GLOOK-8

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