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/components/ConfigSidebar.svelte

@@ -51,6 +51,7 @@
     budget_action: "warn",
     budget_warning_threshold: 0.8,
     discord_rpc_enabled: true,
+    show_thinking_blocks: true,
   });
 
   let showCustomThemeEditor = $state(false);
@@ -703,6 +704,22 @@
           Use Ctrl++ / Ctrl+- to quickly adjust, Ctrl+0 to reset
         </p>
       </div>
+
+      <!-- Show Thinking Blocks Toggle -->
+      <div class="mb-4">
+        <label class="flex items-center gap-3 cursor-pointer">
+          <input
+            type="checkbox"
+            bind:checked={config.show_thinking_blocks}
+            class="w-4 h-4 rounded border-[var(--border-color)] bg-[var(--bg-primary)] text-[var(--accent-primary)] focus:ring-[var(--accent-primary)]"
+          />
+          <span class="text-sm text-[var(--text-primary)]">Show Extended Thinking Blocks</span>
+        </label>
+        <p class="text-xs text-[var(--text-tertiary)] mt-1 ml-7">
+          Display Claude's extended thinking process in the conversation. Thinking blocks can be
+          expanded/collapsed to see reasoning details.
+        </p>
+      </div>
     </section>
 
     <!-- Window Section -->