feat(electrobun): redesign AgentPane to match Claude Code VSCode extension
Based on official Claude Code v2.1.79 webview CSS analysis:
- Timeline pattern: 7px dots + 1px vertical line, 30px content indent
- User messages: left-aligned inline blocks (not right-aligned bubbles)
- Tool calls: flat bordered grid boxes with 60px mask-fade clipping
- Floating input: absolute bottom:16px, crust bg, 8px radius, shadow
- ChatInput.svelte extracted: auto-resize textarea, peach send button
(26×26, 5px radius), focus ring with color-mix peach 12%
- Footer strip: model name + attach button + divider + send button
- Status strip: compact top bar with dot + status + model + cost
- 150px gradient fade at message area bottom
- fadeIn 0.3s animation on new messages
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