[Improve] Keep shared controls legible across IDE themes (#1333)

* test(webview): add WCAG contrast audit

* test(webview): add contrast snapshot baselines

* test(webview): harden contrast assertions

* test(webview): refresh resting contrast baselines

* test(webview): refresh focus contrast baselines

* fix(webview): strengthen field focus contrast

* test(webview): capture accessible focus baselines

* test(webview): assign visual fixtures to CT coverage

* docs(webview): document CI visual baseline fallback

* test(webview): snapshot production chat and UI settings

* fix(webview): scope visual test type shim

* fix(webview): align contrast tests with modern themes

* test(webview): add pinned production baselines

* test(webview): refresh modern theme baselines

* test(webview): refresh light dialog baseline

* test(webview): isolate settings theme surfaces

* test(webview): refresh light settings baselines

* test(webview): generate themes from pinned VS Code

* test(webview): refresh generated theme baselines

* test(webview): align generated theme expectations

* test(webview): refresh interactive theme states

* test(webview): refresh focused theme baselines

* test(webview): refresh dark dialog baseline

* ci(webview): check generated theme drift

* test(webview): stabilize generated theme fonts

* test(webview): normalize fixture typography

* test(webview): cover theme fixture validation

* refactor(theme-fixtures): move theme probe off the public API

* fix(webview): restore button mock styles lost in theme migration

* test(webview): cover theme probe lifecycle

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Co-authored-by: Roomote <roomote@roomote.dev>
Co-authored-by: Elliott de Launay <edelaunay@wealthsimple.com>