feat: add extended thinking blocks display

Implements support for displaying Claude's extended thinking blocks in
the conversation UI with a collapsible, visually distinct component.

Changes:
- Backend: Update wsl_bridge.rs to emit thinking blocks with dedicated
  line_type instead of system messages
- Types: Add "thinking" to TerminalLine type union
- Config: Add show_thinking_blocks toggle (default: true)
- UI: Create ThinkingBlock.svelte component with collapsible interface
- Terminal: Update to conditionally render thinking blocks
- Settings: Add toggle in Appearance section of ConfigSidebar

The ThinkingBlock component features:
- Lightbulb icon to indicate extended thinking
- Collapsible/expandable with animated chevron
- Distinct styling: dimmed, italic, monospace
- Timestamp display
- Respects global show_thinking_blocks config setting

Note: Extended thinking support in Claude Code CLI appears to be in
development. This implementation is ready and will automatically display
thinking blocks once the CLI begins emitting them.

Issue: #120

✨ Implemented by Hikari~ 🌸

src-tauri/src/wsl_bridge.rs

@@ -974,8 +974,8 @@ fn process_json_line(
                         let _ = app.emit(
                             "claude:output",
                             OutputEvent {
-                                line_type: "system".to_string(),
-                                content: format!("[Thinking] {}", thinking),
+                                line_type: "thinking".to_string(),
+                                content: thinking.clone(),
                                 tool_name: None,
                                 conversation_id: conversation_id.clone(),
                                 cost: None,