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/lib/stores/config.test.ts

@@ -193,6 +193,7 @@ describe("config store", () => {
         budget_action: "warn",
         budget_warning_threshold: 0.8,
         discord_rpc_enabled: true,
+        show_thinking_blocks: true,
       };
 
       expect(config.model).toBe("claude-sonnet-4");
@@ -238,6 +239,7 @@ describe("config store", () => {
         budget_action: "warn",
         budget_warning_threshold: 0.8,
         discord_rpc_enabled: true,
+        show_thinking_blocks: true,
       };
 
       expect(config.model).toBeNull();
@@ -773,6 +775,7 @@ describe("config store", () => {
         budget_action: "block",
         budget_warning_threshold: 0.9,
         discord_rpc_enabled: false,
+        show_thinking_blocks: true,
       };
 
       const mockInvokeImpl = vi.mocked(invoke);