feat: batch of fixes and features (#56)

## Summary

This PR includes a batch of bug fixes and new features:

### Bug Fixes
- **Links in chat history now open in default browser** instead of navigating within the app
  - Closes #54
- **Allow spaces in tab names** - space key no longer acts like enter when renaming tabs
  - Closes #52

### New Features
- **`/cd` command** - Change the working directory of an active tab with context preservation
  - Closes #55
- **`/search` command** - Search and highlight matches within the conversation
  - Closes #32

## Test Plan
- [ ] Click a link in chat history and verify it opens in the default browser
- [ ] Rename a tab and verify spaces can be typed
- [ ] Use `/cd <path>` and verify the directory changes while preserving conversation context
- [ ] Use `/search <query>` and verify matches are highlighted in yellow
- [ ] Use `/search` with no args to clear the search highlighting

✨ This PR was created with help from Hikari~ 🌸

Co-authored-by: Naomi Carrigan <commits@nhcarrigan.com>
Reviewed-on: #56
Co-authored-by: Hikari <hikari@nhcarrigan.com>
Co-committed-by: Hikari <hikari@nhcarrigan.com>

src/lib/components/ConversationTabs.svelte

@@ -126,6 +126,8 @@
     } else if (event.key === "Escape") {
       editingTabId = null;
       editingName = "";
+    } else if (event.key === " ") {
+      event.stopPropagation();
     }
   }