0.5.0

0.5.0 — Insert Pair, Ctrl+wheel zoom, sidebar sync, study-mode keys

New

• Insert Pair. Inserts a new pair number, shifting every shape with a
  >= target pair number up by 1 so a fresh group slot opens at the chosen
  position. The current shape is excluded from the shift so the active
  selection isn't accidentally moved.
• Ctrl+wheel zooms toward the cursor when scrolling inside the content
  frame, so the area under the pointer stays under the pointer as you
  zoom — much faster than zoom-from-center for picking out small labels.
• Sidebar thumb syncs to the main scroll position (one-way,
  PowerPoint-style). As you scroll the content pane the sidebar tracks
  along so the focused image's thumbnail stays visible. Anchored to "top"
  rather than nearest so the highlighted thumbnail consistently sits at
  the start of the visible sidebar window.
• ±1-step Reveal / Hide buttons in the header for explicit single-step
  control without scrolling or keyboard.

Behavior changes

• Arrow up/down always step Reveal/Hide in study mode, regardless of
  the zoom setting or whether arrow-key zoom is enabled. Outside study
  mode, the existing zoom/reveal arrow bindings still apply.
• Hidden covers no longer fade with progress. They're supposed to
  conceal, so the progress-driven opacity decay was working against the
  feature. The rail itself still fades subtly to signal progress.

Infrastructure

• Migrated from npm to pnpm.
• _archive/ is now gitignored.