feat: add gallery page layouts (#670)

[zaidahmad16]

Summary

• Adds /gallery landing page showing collection cards (cover image, title, photo count) and a latest photos grid
• Adds individual collection pages (/gallery/<event>/) showing a full photo grid
• Adds shared gallery-photo-grid.html partial used by both layouts
• Adds gallery CSS for responsive grid layout (single column at ≤360px, multi-column on desktop)
• Commits fixture content tree (3 placeholder collections) for local dev testing

Implementation Notes

• Images processed via Hugo's image pipeline: .Fill "500x500 webp q85" for thumbnails, .Resize "1600x webp q82" for full images
• Collection cover uses .Resources.GetMatch "cover.*" with fallback to first image alphabetically
• All thumbnails have loading="lazy" and explicit width/height
• Wave 1: clicking a photo navigates directly to the full-size image (no lightbox)
• /gallery has not been added to the navbar (data/routes.yml) — left as a product decision for the reviewer

[github-actions]

✅ Preview is ready!

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	📝 Commit	45b6c9a
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	📦 Size	270.1 MB · 1773 files
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[github-actions]

Images automagically compressed by Calibre's image-actions ✨

Compression reduced images by 65.5%, saving 237.1 KB.

Filename	Before	After	Improvement	Visual comparison
content/gallery/ciena-networking-night-2026/carletonLogo.jpg	107.5 KB	40.5 KB	62.4%	View diff
content/gallery/ciena-networking-night-2026/cienaNetworkingNight.png	91.0 KB	31.0 KB	65.9%	View diff
content/gallery/comp-final-study-sessions-2026/COMP_finalStudySession.png	83.4 KB	26.3 KB	68.4%	View diff
content/gallery/end-of-year-bbq-2026/EOY_BQQ.png	77.0 KB	25.1 KB	67.4%	View diff
content/gallery/ciena-networking-night-2026/cienaLogo.png	3.2 KB	2.1 KB	34.1%	View diff

4 images did not require optimisation.