fix(a11y): add optional accessible-name overrides

[BrianGenisio]

Summary

Closes #20 (D9 from the ChatCPT WCAG 2.2 AA audit).

Modal and slider now accept optional accessible-name overrides so consumers can pass already-translated strings. English defaults stay when the options are omitted. There is no i18n runtime in the design system.

Changes

• Modal: closeButtonLabel (default 'Close modal')
• Slider: handleLabel, minHandleLabel, maxHandleLabel (defaults 'Value', 'Minimum value', 'Maximum value')
• Dropdown: no new option. placeholder is already the combobox accessible name when nothing is selected; README / agents.md now say to pass a translated string.

Existing constructors keep working. ChatCPT should pass t(...) after the submodule bump (settings close button, model dropdown placeholder). The temperature slider already replaces aria-label with aria-labelledby, so it does not need handleLabel.

This PR is independent of #24 (D8/D10/D11). Both touch modal.js / the modal README, so rebase this branch if #24 lands first.

Test plan

• Modal default close label is Close modal; override is used when provided
• Slider default handle labels stay English; translated overrides apply to single and range
• Dropdown default placeholder is Select option; custom placeholder is the combobox name
• npm test (existing slider / dropdown / contrast suites unchanged)

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📥 Commits

Reviewing files that changed from the base of the PR and between d21d15d and 3baee8c.

📒 Files selected for processing (7)

• agents.md
• components/dropdown/README.md
• components/modal/README.md
• components/modal/modal.js
• components/numeric-slider/README.md
• components/numeric-slider/numeric-slider.js
• tests/accessible-names.spec.js

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