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.ts

@@ -45,6 +45,8 @@ export interface HikariConfig {
   budget_warning_threshold: number;
   // Discord RPC settings
   discord_rpc_enabled: boolean;
+  // Thinking blocks settings
+  show_thinking_blocks: boolean;
 }
 
 const defaultConfig: HikariConfig = {
@@ -84,6 +86,7 @@ const defaultConfig: HikariConfig = {
   budget_action: "warn",
   budget_warning_threshold: 0.8,
   discord_rpc_enabled: true,
+  show_thinking_blocks: true,
 };
 
 function createConfigStore() {
@@ -297,6 +300,10 @@ export const shouldHidePaths = derived(
   configStore.config,
   ($config) => $config.streamer_mode && $config.streamer_hide_paths
 );
+export const showThinkingBlocks = derived(
+  configStore.config,
+  ($config) => $config.show_thinking_blocks
+);
 
 /**
  * Masks file paths in text when streamer mode with hide paths is enabled.