## ADDED Requirements

### Requirement: Error boundary on MarkdownRenderer

`MarkdownRenderer` SHALL be wrapped in a `<MessageBoundary>` that catches render errors from remark-gfm or react-markdown and shows a compact "Content rendering failed" fallback with a retry button.

#### Scenario: Markdown render throws
- **WHEN** `react-markdown` or `remark-gfm` throws during rendering
- **THEN** the error boundary catches the exception and renders a fallback UI
- **AND** the fallback shows "Content rendering failed" with a Retry button
- **AND** clicking Retry re-mounts `MarkdownRenderer` with the same content

### Requirement: Error boundary on CodeBlock

`CodeBlock` SHALL be wrapped in a `<MessageBoundary>` that catches errors from Shiki's `codeToHtml` and renders a plain-text `<pre>` fallback with the source code.

#### Scenario: Shiki highlight fails
- **WHEN** `codeToHtml` throws (e.g., unknown language, WASM load failure)
- **THEN** the error boundary catches the exception and renders a plain `<pre>` block with the original code

### Requirement: Loading skeleton for streaming messages

Messages with `status === 'streaming'` and no content yet SHALL render a pulse-animated skeleton placeholder instead of an empty bubble.

#### Scenario: Streaming message starts with no content
- **WHEN** a `message_started` frame arrives with `role: 'assistant'`
- **THEN** a skeleton placeholder renders (animated pulse bar) until the first `delta` frame arrives with content

### Requirement: Keyboard-navigable ToolCallLine

`ToolCallLine` SHALL support full keyboard navigation: `Tab` to focus, `Enter`/`Space` to toggle expand/collapse, `Escape` to collapse if expanded.

#### Scenario: User navigates tool call via keyboard
- **WHEN** the user presses `Tab` to focus a `ToolCallLine`
- **THEN** a visible focus ring appears
- **AND** pressing `Enter` toggles expand/collapse
- **AND** pressing `Escape` collapses it if expanded

### Requirement: Keyboard-navigable ToolCallGroup

`ToolCallGroup` SHALL support the same keyboard navigation as `ToolCallLine`. The group header SHALL be the focusable element.

#### Scenario: User navigates tool group via keyboard
- **WHEN** the user presses `Tab` to focus a `ToolCallGroup` header
- **THEN** a visible focus ring appears
- **AND** pressing `Enter` toggles expand/collapse of the group

### Requirement: Consistent aria-label protocol

Every interactive element in `ActionRow` (Copy, Resend, Regenerate, Fork, Delete, Restore, Pin) SHALL have an `aria-label` attribute that matches its `title` text.

#### Scenario: Action row button has aria-label
- **WHEN** an ActionRow button renders
- **THEN** it SHALL have an `aria-label` matching its `title`
- **AND** the label SHALL be unique within the message bubble
