v6: Make the font-size setting reach the doc explorer, headers, and pills

[trevor-scheer]

Summary

• The doc-explorer's field-card, type-card, and fields-list all hardcoded font-size in px (11.5px, 14px, 10px, 9px, and more), so none of that text grew or shrank with the font-size preset. They now read --font-size-mono, --font-size-small, or --font-size-eyebrow depending on which is closest to the original size.
• response-header, panel-header, and method-pill had the same problem: fixed px values that ignored Settings > Font size entirely. Migrated to the matching tokens.
• The legacy --font-size-hint token (root.css) was a frozen value that never scaled with the preset, at ~19 call sites across var-headers-strip, the collections plugin, the history plugin, and the doc-explorer plugin. Every usage now points at --font-size-eyebrow instead, and the dead --font-size-hint definition is gone.
• Along the way, found that --font-size-body itself is silently shadowed by a frozen legacy value in root.css (same class of bug, but out of scope here, so flagged separately). Avoided that token in the new mappings and used --font-size-mono for the mono-family names/types instead, since it actually rescales.

Test plan

• Open Settings, set Font Size to Large or Extra Large, then open the Documentation Explorer. Confirm type names, field names/types, descriptions, and the "FIELDS · N" section label all grow with it (they previously stayed fixed).
• Run a query and confirm the response header (status code, timing, size) grows with the font-size setting.
• Confirm panel headers (e.g. the "History" and "Documentation Explorer" panel titles) grow with the setting.
• Confirm method pills (QRY/MUT/SUB badges) grow with the setting.
• Set the font size back to Default and confirm everything returns to its original size with nothing shifted or clipped.
• Check both Dark and Light themes at a couple of font-size presets to confirm nothing overlaps or truncates awkwardly at the larger sizes.

Refs: #4228

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