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-tauri/src/commands.rs

@@ -105,6 +105,51 @@ pub async fn get_usage_stats(
     manager.get_usage_stats(&conversation_id)
 }
 
+#[tauri::command]
+pub async fn validate_directory(path: String, current_dir: Option<String>) -> Result<String, String> {
+    use std::path::Path;
+
+    let path = Path::new(&path);
+
+    // Expand ~ to home directory
+    let expanded_path = if path.starts_with("~") {
+        if let Some(home) = std::env::var_os("HOME") {
+            let home_path = Path::new(&home);
+            if path == Path::new("~") {
+                home_path.to_path_buf()
+            } else {
+                home_path.join(path.strip_prefix("~").unwrap())
+            }
+        } else {
+            return Err("Could not determine home directory".to_string());
+        }
+    } else if path.is_relative() {
+        // Handle relative paths (., .., or any relative path) by resolving against current_dir
+        if let Some(ref cwd) = current_dir {
+            Path::new(cwd).join(path)
+        } else {
+            path.to_path_buf()
+        }
+    } else {
+        path.to_path_buf()
+    };
+
+    // Check if the path exists and is a directory
+    if !expanded_path.exists() {
+        return Err(format!("Directory does not exist: {}", expanded_path.display()));
+    }
+
+    if !expanded_path.is_dir() {
+        return Err(format!("Path is not a directory: {}", expanded_path.display()));
+    }
+
+    // Return the canonicalized (absolute) path
+    expanded_path
+        .canonicalize()
+        .map(|p| p.to_string_lossy().to_string())
+        .map_err(|e| format!("Failed to resolve path: {}", e))
+}
+
 #[tauri::command]
 pub async fn load_saved_achievements(app: AppHandle) -> Result<Vec<AchievementUnlockedEvent>, String> {
     use chrono::Utc;