feat: variant-aware code pane in the code block's own style

[divyanshub024]

Summary

Reworks the playground's code panel so it reads like the package instead of like VS Code, and keeps its snippet in step with the stage:

• The snippet is typeset in the code block's own style: the package's built-in FlowSyntaxHighlighter over the theme's FlowSyntaxColors and Geist Mono code role, directly on the panel's ground with no card chrome. The syntax_highlight dependency, its bundled grammars, and the startup init are gone.
• Snippets follow the variant pills. snippetFor takes the stage's active variant and every demo now ships per-variant code: composer streaming, message roles, each code block language, all four markdown variants, error card/minimal/failed turn, pill forms, attachments, thread default/streaming/short, and the on/off pairs for the streaming, shimmer, thinking, suggestions and greeting demos.
• The panel resizes by dragging its left edge, clamped between the default 400px and 720px; double-tap snaps it back. The width tracks the pointer directly, with the slide easing reserved for open and close.
• Copy and close sit together as matching header chips; copy is always visible and flips to a primary-tinted check while the confirmation lasts, resetting when the demo or variant changes.

The highlighter is not part of the public barrel, so the panel reaches into package:flow_ui/src/ for it with a commented, lint-acknowledged import. The playground absorbs that coupling instead of the package growing panel-only API.

Screenshots

Before	After

How this was verified

flutter analyze clean in playground/ and the package root, dart format applied, and a full flutter build web compile. Exercised the panel in the running playground: variant pills swap the snippet, drag resize with clamping and double-tap reset, copy confirmation, and both themes.

Checklist

• flutter analyze lib and flutter analyze in example/ and playground/ are clean
• dart format . applied
• Exercised in the playground — with a stage demo added or updated if this is a new component or variant
• No new entries under dependencies: in pubspec.yaml (Flutter SDK and flutter.dev packages only)
• Nothing model-facing — no prompts, schemas, or provider/network calls
• New public API is exported from lib/flow_ui.dart and documented in docs/ and the README table
• CHANGELOG.md updated for user-facing changes, with breaking changes called out
• PR title follows conventional commits (feat:, fix:, refactor:, docs:, chore:)

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Note

Low Risk
Playground-only UI and demo snippet changes; no package public API or runtime product paths.

Overview
Reworks the playground code panel so snippets look like FlowCodeBlock and stay in step with the stage.

Drops syntax_highlight (and its async grammar init) in favor of the package’s built-in FlowSyntaxHighlighter over theme syntax colors and the code typeface. The panel reaches into flow_ui/src for that highlighter rather than expanding public API.

snippetFor now takes the active variant; demos ship per-pill snippets (composer streaming, message roles, code languages, markdown/error/pill/thread forms, and on/off pairs). The pane is drag-resizable (400–720px, double-tap reset), with copy/close as matching header chips.

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