Epic: UI/UX & docs polish

[ApocDev]

Cross-cutting polish on the web app's look, responsiveness, and documentation, distinct from the feature work that adds new surfaces.

Sub-issues:

• Responsive/mobile: make the app usable on small screens #9 — responsive and small-screen usability.
• Flesh out the in-app help drawers (images, worked examples, deeper coverage) #16 — flesh out the in-app help drawers (images, worked examples, deeper coverage).
• Holistic UX/UI design pass across the web app #17 — holistic UX/UI design pass across the web app.