feat: add package trends comparison and chart export

[pastelsky]

Summary

The package-trends feature, stacked on #1080. Review and merge that foundations PR first; this PR then contains only the trends route, API, data collection, rendering, and feature-specific styling.

What changed

• Compare package downloads, cached bundle-size history, and current GitHub repository snapshots at /trends.
• Add supported ranges, grouping, release overlays, recommendation shortcuts, magnetic chart inspection, and accessible package snapshots.
• Provide /api/trends and /api/trends-image with cache-backed data collection and share/export support.
• Add the Trends navigation link and feature-scoped tooltip/layout tokens.

Validation

• yarn test __tests__/trends.test.ts __tests__/trendsRecommendations.test.ts --runInBand (16 passed)
• yarn build
• Earlier Playwright desktop and 390px mobile checks passed; rerun once the stacked PR is mergeable.

[pastelsky]

Visual evidence — package trends

Captured against the stacked trends branch. It covers loading, data-bearing states, interaction variants, themes, and responsive navigation.

Initial and chart states

Package and theme variants

Mobile states

[pastelsky]

Trends suggestion composition

The trends route now owns related-package suggestion composition. It passes loadRelatedPackageSuggestions to the shared autocomplete, rather than passing related package names into it.

rankSuggestionsByQuery is a pure trends helper: it deduplicates merged results, removes non-matches, places prefix matches first, then applies the shared search-score ordering. The shared autocomplete and all existing consumers continue to use a single suggestion loader.