feat: add Typst lesson template with Jellyfin media frontmatter

- Add templates/lesson.typ: reusable template with lesson-template() function
  - Accepts title, grade, subject, date, media, description parameters
  - Comment-based frontmatter convention documented at top
  - Renders title heading, metadata line, description, body, media section
  - Media references use raw(lang: "jellyfin-media") for identifiable HTML elements
- Add templates/example-lesson.typ: complete example (Water Cycle lesson)
  - Demonstrates all template features
  - Christian-homeschool appropriate content (Psalm 135:7)
  - Includes 2 Jellyfin media references

Closes #3

templates/example-lesson.typ

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+// example-lesson.typ — Complete example demonstrating the lesson template
+// ============================================================
+// Frontmatter:
+// title: God's Creation: The Water Cycle
+// grade: 3
+// subject: Science
+// date: 2026-01-15
+// media: jellyfin://uuid-water-cycle, jellyfin://uuid-cloud-formation
+// description: Learn how God designed water to move through His creation —
+//   evaporation, condensation, and precipitation working together to sustain life.
+
+#import "lesson.typ": lesson-template, media-ref
+
+#show: doc => lesson-template(
+  doc,
+  title: "God's Creation: The Water Cycle",
+  grade: "3",
+  subject: "Science",
+  date: "2026-01-15",
+  media: (
+    ("uuid-water-cycle", "The Water Cycle Explained"),
+    ("uuid-cloud-formation", "Cloud Formation and Weather"),
+  ),
+  description: "Learn how God designed water to move through His creation — evaporation, condensation, and precipitation working together to sustain life on Earth.",
+)
+
+= What Is the Water Cycle?
+
+Water is one of God's most amazing creations! The water cycle is how water moves
+all around our planet. It never runs out — it just keeps going around and around.
+There are four main steps: evaporation, condensation, precipitation, and collection.
+
+== Evaporation
+
+When the sun shines on oceans, lakes, and rivers, the water gets warm and turns
+into a gas called *water vapor*. This gas rises up into the sky. You can't see it,
+but it's there! Evaporation happens every day, especially on warm, sunny days.
+
+== Condensation
+
+When the water vapor goes high up where it's cold, it turns back into tiny water
+droplets. These droplets come together to make clouds. Have you ever looked at
+clouds and seen shapes? Those shapes are made of millions of tiny water droplets
+sticking together!
+
+== Precipitation
+
+When the clouds get too heavy with water droplets, the water falls back down to
+Earth. This is called *precipitation*. It can be:
+
+- Rain — liquid water falling from the sky
+- Snow — frozen crystals that fall in cold weather
+- Sleet — tiny ice pellets that bounce when they land
+- Hail — hard balls of ice that form in big storms
+
+God sends precipitation to water the plants and fill the rivers, lakes, and oceans.
+
+== Collection
+
+The water that falls onto the ground collects in rivers, lakes, and oceans. Some
+of it soaks into the ground and becomes *groundwater* — water stored underground
+that plants can reach with their roots. Then the sun warms the water, and the
+whole cycle starts all over again!
+
+= Why Is the Water Cycle Important?
+
+God designed the water cycle to do important work:
+
+- Give plants the water they need to grow
+- Provide fresh water for animals and people to drink
+- Keep our rivers and lakes full of water
+- Help control the temperature of our whole planet
+- Clean the water naturally as it moves through the cycle
+
+*"He causes the vapors to ascend from the ends of the earth; He makes lightning
+for the rain; He brings the wind out of His treasuries."* — Psalm 135:7
+
+= Think About It
+
+1. What happens to water when the sun shines on a lake?
+2. Why do clouds form high up in the sky?
+3. What are the four types of precipitation?
+4. How does the water cycle show God's care for His creation?

templates/lesson.typ

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+// lesson.typ — Typst template for homeschool lessons
+// ============================================================
+// This template produces clean, structured lesson documents.
+// Designed to compile to HTML for rendering inside Jellyfin.
+//
+// Frontmatter convention (documented as comments at top of each lesson):
+//   // title: The Lesson Title
+//   // grade: 3
+//   // subject: Science
+//   // date: 2026-01-15
+//   // media: jellyfin://uuid-1, jellyfin://uuid-2
+//   // description: Short description of the lesson
+//
+// Usage:
+//   #import "lesson.typ": lesson-template, media-ref
+//   #show: doc => lesson-template(
+//     doc,
+//     title: "The Water Cycle",
+//     grade: "3",
+//     subject: "Science",
+//     date: "2026-01-15",
+//     media: (),
+//     description: "Learn about evaporation and condensation.",
+//   )
+
+// Render a single Jellyfin media reference.
+// Produces a <code data-lang="jellyfin-media"> element in HTML output
+// that the Jellyfin plugin can parse and link to the player.
+#let media-ref(id, title) = {
+  raw(lang: "jellyfin-media", id + "|" + title)
+}
+
+// Render a list of media references as a formatted section.
+#let media-section(media-items) = {
+  if media-items.len() > 0 {
+    heading(level: 2)[Media]
+    for item in media-items {
+      list.item(media-ref(item.at(0), item.at(1)))
+    }
+  }
+}
+
+// The main lesson template.
+// - body: The lesson content (passed via show rule)
+// - title: Lesson title (string)
+// - grade: Grade level (string, e.g. "3")
+// - subject: Subject name (string)
+// - date: Lesson date in YYYY-MM-DD format
+// - media: Array of (id, title) pairs for linked Jellyfin media
+// - description: Short description of the lesson
+#let lesson-template(
+  body,
+  title: "Untitled Lesson",
+  grade: none,
+  subject: none,
+  date: none,
+  media: (),
+  description: none,
+) = {
+  // Document metadata for PDF output
+  set document(title: title)
+
+  // Title — prominent heading
+  heading(level: 1, title)
+
+  // Metadata line: Grade X — Subject — Date
+  let meta-parts = ()
+  if grade != none { meta-parts.push([Grade ] + grade) }
+  if subject != none { meta-parts.push(subject) }
+  if date != none { meta-parts.push(date) }
+  if meta-parts.len() > 0 {
+    let sep = [ — ]
+    let meta-text = meta-parts.first()
+    for i in range(1, meta-parts.len()) {
+      meta-text += sep + meta-parts.at(i)
+    }
+    par(text(style: "italic", size: 0.9em, meta-text))
+  }
+
+  // Description paragraph
+  if description != none {
+    par([*Description:* ] + description)
+  }
+
+  // Lesson body content (renders user-authored sections, paragraphs, lists, etc.)
+  body
+
+  // Media references section — listed with identifiable jellyfin-media codes
+  media-section(media)
+}