Rename fireflies → sprites across codebase and UI

[thalida]

Rename the fireflies world element to sprites consistently across the codebase and UI. The glowing orbs that orbit the commit trees (one per author, color = author identity, size = commit count) should be called "sprites" everywhere.

Why

"Sprites" was preferred over "fireflies" (and over "spirits") as the canonical name for these author-orbs. Right now "firefly/fireflies" is the term throughout code, settings, and UI. This is a global vocabulary change to make the naming consistent.

Scope

Blast radius (at time of writing): ~129 references across 21 files, plus a directory and file/shader names. A pure rename — no behavior change.

• Component directory & files: city/components/fireflies/ → city/components/sprites/, including firefliesRenderer.ts, firefliesPlacement.ts, fireflies.ts, fireflies.frag.glsl, fireflies.vert.glsl, index.ts, authorColor.ts, orbitRings.ts.
• Settings store + schema: state/stores/settings/fireflies.ts, the FIREFLIES namespace in state/settingsSchema.ts, and the section declaration views/ControlsPane/partials/Fireflies.ts.
• UI labels: the "Fireflies" Settings section, and any references in CommitPane.tsx.
• Cross-references: city/index.ts, city/state/index.ts, city/types/renderOrders.ts, trees (treeRenderer.ts, treeEncoding.ts, trees/index.ts), pathLine/index.ts, rainbowChase.ts, ControlsPane.tsx.
• Identifiers, types, CSS classes, and comments — all firefly/fireflies → sprite/sprites.

Watch out for

• Persisted settings keys — if the FIREFLIES settings namespace is persisted to localStorage, renaming the key will reset users' saved sprite settings to defaults. Decide whether to migrate the stored key or accept the reset (per the project's no-backwards-compat stance, a reset is likely acceptable — confirm).
• The World Almanac (Add metadata about the world #60) already uses "Fireflies" as the section label; after this rename it becomes "Sprites." Sequence accordingly if Add metadata about the world #60 lands first.

Out of scope

• Any change to how the orbs are rendered or what data drives them — name only.