Add hover/focus popovers for whitepaper footnotes and citations

[jubalm]

Context

The public whitepaper pages now preserve two different reference systems:

• Footnotes: same-page explanatory notes, such as superscript references linked to footnote text.
• In-text citations: bracketed references such as [1–3], which point to numbered entries on the dedicated References page.

Proposed scope

Add a shared, accessible popover primitive and two reference adapters:

• FootnotePopover: preview same-page footnote text on hover/focus, while preserving the normal link and footnotes for keyboard, touch, printing, and no-JavaScript use.
• CitationPopover: resolve individual citations and ranges such as [1–3] to stable anchors on the References page, showing a compact list of linked reference entries.

Requirements

• Progressive enhancement; the document remains usable without JavaScript.
• Keyboard focus support and sensible dismissal behavior.
• Tap-friendly behavior on touch devices.
• Viewport-aware positioning near page edges.
• Stable reference anchors and optional return links from the References page.
• Handle citation ranges and repeated/individual citations.

Deliberately out of scope for PR #177

This should be implemented as a follow-up after the whitepaper page structure and navigation work in PR #177 are reviewed.