feat(ui): searchable model picker with provider group headers (PR #659 by @mmartial)

[nesquena-hermes]

Searchable model picker (rebase-on-behalf of PR #659 by @mmartial)

This is a clean rebase of @mmartial's #659 onto current master, plus two correctness fixes that were found and addressed during review.

What this adds

• Live search input at the top of the model dropdown — type any part of a model name or ID to filter the list instantly
• Provider group headers preserved in filtered results (Anthropic, GitHub Copilot, OpenAI Codex, OpenRouter headers remain visible while filtering)
• Clear button (✕) resets search and restores full list
• Escape key closes the dropdown (tested and confirmed by @mmartial)
• "No models found" empty state shown when nothing matches
• i18n — EN, ES, and zh-CN strings for the two new keys

Fixes applied during review

1. zh-CN double-escape bug (commit baab684 on the original PR, reviewed by @nesquena): four zh-CN strings in i18n.js were double-escaped (\\u81ea instead of \u81ea), causing raw \u... text to render in the Chinese UI. Fixed.

2. Provider group headers regression (identified during integration QA): the initial implementation flattened all models into a _modelData array without storing the group label, so _filterModels() rendered a flat unlabeled list. Fixed by storing group on each entry and emitting .model-group headings in _filterModels() when the group changes.

3. CSS theme compatibility: the original PR used rgba(124,185,255,.5) for focus border color. Current master uses var(--accent) throughout (from the theme system in PR Replace color scheme system with light/dark theme + accent skins #627). Updated to match.

Test results

• 1423 tests pass, 0 failures
• node --check passes for static/ui.js and static/i18n.js
• Browser QA: search filters correctly, group headers visible, clear button works, Escape closes, no-results state shown

Closes #659
Co-authored-by: mmartial mmartial@users.noreply.github.com