Theme react-select and react-dropdown-tree-select to Tokyo-11

[WilfordGrimley]

Follow-up from the 2026-07-25 AAA contrast sweep (PR #456). Owner approved 2026-07-25.

Two third-party components render their own unthemed white/grey defaults, outside our token layer:

• react-select
• react-dropdown-tree-select

They are currently allowlisted in the new ContrastAudit regression gate (logged, not silently dropped) — this issue removes them from the allowlist by fixing them.

Likely connected to a standing owner complaint: the "grey sources, too much vertical space" feedback on the source-filter panel. That panel is the dropdown-tree-select, so theming it may resolve both the contrast failure and the visual complaint in one pass. Confirm before assuming.

Requirements:

• Theme both libraries to Tokyo-11 tokens (both expose styling APIs — react-select has the styles/classNames props; react-dropdown-tree-select is CSS-class based). Route from _theme-tokens.scss rather than hardcoding values.
• Meet the ratified AAA bar: 7:1 normal text, 4.5:1 large/bold. Cover menu open/closed, hover, selected, focused, and disabled option states.
• Remove the two allowlist entries from the ContrastAudit gate once passing.
• Sequenced AFTER PR Fix accordion/card-body contrast and outline-secondary legibility (Tokyo-11 sweep) #456 merges to avoid two frontend branches colliding in styles.scss.

Do not start until #456 has landed.