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/HighlightedText.svelte

@@ -0,0 +1,61 @@
+<script lang="ts">
+  export let content: string;
+  export let searchQuery: string;
+
+  interface TextPart {
+    text: string;
+    isMatch: boolean;
+  }
+
+  function getHighlightedParts(text: string, query: string): TextPart[] {
+    if (!query) {
+      return [{ text, isMatch: false }];
+    }
+
+    const escapedQuery = query.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
+    const regex = new RegExp(`(${escapedQuery})`, "gi");
+    const parts: TextPart[] = [];
+    let lastIndex = 0;
+    let match: RegExpExecArray | null;
+
+    while ((match = regex.exec(text)) !== null) {
+      // Add non-matching text before the match
+      if (match.index > lastIndex) {
+        parts.push({
+          text: text.slice(lastIndex, match.index),
+          isMatch: false,
+        });
+      }
+
+      // Add the matching text
+      parts.push({
+        text: match[1],
+        isMatch: true,
+      });
+
+      lastIndex = regex.lastIndex;
+    }
+
+    // Add any remaining text after the last match
+    if (lastIndex < text.length) {
+      parts.push({
+        text: text.slice(lastIndex),
+        isMatch: false,
+      });
+    }
+
+    return parts;
+  }
+
+  $: parts = getHighlightedParts(content, searchQuery);
+</script>
+
+<span class="whitespace-pre-wrap">
+  {#each parts as part, index (index)}
+    {#if part.isMatch}
+      <mark class="search-highlight">{part.text}</mark>
+    {:else}
+      {part.text}
+    {/if}
+  {/each}
+</span>