Add shared dark-surface color tokens to theme-styles.ts

[iisa]

Problem

The house palette in src/themes/theme-styles.ts is light-theme oriented — --primary-text-color (dark-gray) sits on --primary-background-color (off-white), and the darkest base color is --dark-gray (#2c2c2c). Dark-surface components can't consume the semantic tokens without inverting them, so they carry local hex defaults instead.

Surfaced during the item-navigator migration (PR #64). ia-item-navigator is a dark UI (white text/icons on near-black) and currently re-declares these hexes because the palette has no equivalent:

• near-blacks #000, #151515, #212121 (theater / embed field / drawer)
• border #4b4b4b — sits between --mid-gray (#333) and --light-gray (#666)
• active-file highlight #538bc5 — neither --navy-blue (#194880) nor --bright-blue (#4b64ff)
• translucent overlays rgba(255,255,255,.1) / rgba(255,255,255,.05), scrim rgba(0,0,0,.7)

Sibling elements components (ia-button, ia-otp, ia-status-indicator) all consume the semantic tokens; none reference raw base names. Dark components have no such path today.

Proposal

• Add a small set of shared dark-surface base tokens to theme-styles.ts (e.g. --surface-dark-1/2/3 for the near-black ladder, --border-dark, a --scrim, and an overlay-hover token for the translucent whites).
• Optionally expose matching semantic/adjustable tokens (--ia-theme-* fallbacks) consistent with the existing convention.
• Re-point ia-item-navigator's --item-navigator-* fallbacks at the new tokens, replacing the local hexes.

Out of scope

• ia-item-navigator behavior or layout — this is palette infrastructure only.
• The #538bc5 active-file blue can be revisited here (fold into a shared accent) or left as a component knob; call it out during design.

Tracking

• Jira: WEBDEV-8796
• Surfaced by: item-navigator migration — WEBDEV-8458 / WEBDEV-8458 Migrate item-navigator into elements #64