feat: code block with built-in syntax highlighting

[divyanshub024]

Summary

Roadmap row 21. Adds FlowCodeBlock and a FlowCodePart message part, with copy intent threaded through FlowMessage and FlowThread (onCodeCopy, copiedCodePart, codeCopyTooltip).

Highlighting is built in and synchronous: each language is an ordered regex rule table compiled into one alternation and colored in a single pass. No assets, no init call. Ships dart, json, javascript/typescript, python, bash, yaml, html, css, sql, plain. Hosts add languages with FlowCodeLanguage.register; caseSensitive: false compiles a table like SQL's case-insensitively.

Also in this PR:

• FlowSyntaxColors theme tokens (eight roles, light and dark presets) on FlowTheme.syntax.
• Bundled Geist Mono (3 weights, SIL OFL) behind new code and codeInline typography roles. withFontFamily() no longer touches the mono roles; withCodeFontFamily() swaps them.

Notes for review:

• Why not the syntax_highlight package: its super_clipboard chain adds a native Rust plugin, an Android minSdk 23 floor, and a build-time binary download to every consumer, and its asset grammars force an async init. dependencies: in pubspec.yaml is unchanged.
• While a message streams, a trailing code part renders each delivery whole (no per-character reveal) and hides the copy affordance.
• Unterminated strings mid-stream stay plain until the closing quote arrives. Deliberate.
• Syntax palette and block metrics are placeholders until the Figma file has a frame for this component.
• Shares CHANGELOG, roadmap rows, and flow_message.dart lines with feat: error state with retry #13. Whichever merges second resolves trivial conflicts.

Screenshots

Before	After
Fenced code rendered as literal backticks in plain text	Highlighted block with header label and copy affordance (see the playground's Code Block stage: 8 variants)

How this was verified

Playground Code Block stage on Chrome (web), both themes. All 8 variant pills render (Dart, JSON, YAML, HTML, CSS, SQL colored; Plain uncolored; Streaming grows without restart flicker). Copy click passed the full source through navigator.clipboard.writeText and flashed the check state. Long lines scroll inside the block, not the page. Thread stage shows a code part in an assistant turn with the text reveal above it. Docs site builds clean.

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

Medium Risk
New public message/theme API and a regex highlighter that runs in build on streamed source. withFontFamily() behavior change for mono roles can surprise hosts who already swapped the whole type scale.

Overview
Ships fenced code as a first-class message part: FlowCodeBlock plus FlowCodePart, rendered by FlowMessage/FlowThread with copy intent on onCodeCopy (host owns the clipboard). Unknown languages render plain; streaming trailing code re-highlights whole and hides copy.

Highlighting is a synchronous regex engine (no extra deps): ordered rule tables compiled to one alternation. Built-ins cover Dart, JSON, JS/TS, Python, shell, YAML, HTML, CSS, SQL, and plain; hosts extend via FlowCodeLanguage.register.

Theme adds FlowSyntaxColors (eight token roles, light/dark) and Geist Mono behind new code / codeInline roles. withFontFamily() no longer retargets the mono face — use withCodeFontFamily().

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