ux: small interaction fixes — Back button, "/" shortcut with modifiers, mixed Licence/License spelling, "View" label #106

Why it matters
Four small things a first-time visitor trips over, bundled because each is a one-line fix:

1. Choosing a category or sort never creates a history entry, so Back leaves the site.
2. The "/" focus-search shortcut also fires on Ctrl+/ and Cmd+/, which browsers and extensions use.
3. The UI spells the word both ways on one screen: "License" (index.html:68, :157), "Licence" (index.html:143, entry pages, chips).
4. The perspective select is labelled "View" with the explanation only in a title tooltip, which touch users never see.

Evidence

• site/public/app.js:137 uses history.replaceState for every state change (also subject to Safari's 100-per-30-s throttle, which throws).
• site/public/app.js:716 key handler has no modifier check.
• site/public/index.html:68, 132-157; site/public/app.js:316.

Suggested approach
pushState for category and sort, replaceState for keystrokes, both in try/catch; add !e.ctrlKey && !e.metaKey && !e.altKey; pick "Licence" (the catalog's own spelling in docs) and apply it everywhere in the UI; rename the select to "Camera view" and move the tooltip text into a visible hint or the filter legend.

Done when
Back returns to the previous filter state, Ctrl+/ reaches the browser, one spelling is used in the UI, and the perspective control is understandable without hovering.