feat(web): render write/edit tool output as a diff card

The write/edit tools already declare card:'diff' with applied hunks on
callView/resultView, but the Web client discarded it: a mutation landed on
GenericToolCard and the details panel flattened the result to a <pre>. Add
DiffBlock (ui-primitives), diff-card-model (the single callView/resultView
derivation), and FileMutationRow (keyed under write and edit), and make the
generic fallback row and the details panel diff-aware. The +/- block form,
per-file path header, same-file gap, and footer mirror the TUI diff card;
the chat row caps at CHAT_DIFF_MAX_LINES against the panel's full height.

packages/client/ui-primitives/src/index.ts

@@ -22,6 +22,8 @@ export { JsonTree } from './JsonTree.tsx'
 export type { JsonTreeProps } from './JsonTree.tsx'
 export { TerminalBlock, DEFAULT_TERMINAL_MAX_LINES } from './TerminalBlock.tsx'
 export type { TerminalBlockProps } from './TerminalBlock.tsx'
+export { DiffBlock, DEFAULT_DIFF_MAX_LINES } from './DiffBlock.tsx'
+export type { DiffBlockProps, DiffHunk } from './DiffBlock.tsx'
 export { CodeBlock } from './markdown/CodeBlock.tsx'
 export { JsonBlock } from './markdown/JsonBlock.tsx'
 export { MarkdownText } from './markdown/MarkdownText.tsx'