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/stores/search.ts

@@ -0,0 +1,63 @@
+import { writable, derived } from "svelte/store";
+
+interface SearchState {
+  query: string;
+  isActive: boolean;
+  matchCount: number;
+  currentMatchIndex: number;
+}
+
+const initialState: SearchState = {
+  query: "",
+  isActive: false,
+  matchCount: 0,
+  currentMatchIndex: 0,
+};
+
+const searchStore = writable<SearchState>(initialState);
+
+export const searchState = {
+  subscribe: searchStore.subscribe,
+
+  setQuery: (query: string) => {
+    searchStore.update((state) => ({
+      ...state,
+      query,
+      isActive: query.length > 0,
+      currentMatchIndex: 0,
+    }));
+  },
+
+  setMatchCount: (count: number) => {
+    searchStore.update((state) => ({
+      ...state,
+      matchCount: count,
+    }));
+  },
+
+  nextMatch: () => {
+    searchStore.update((state) => ({
+      ...state,
+      currentMatchIndex:
+        state.matchCount > 0 ? (state.currentMatchIndex + 1) % state.matchCount : 0,
+    }));
+  },
+
+  previousMatch: () => {
+    searchStore.update((state) => ({
+      ...state,
+      currentMatchIndex:
+        state.matchCount > 0
+          ? (state.currentMatchIndex - 1 + state.matchCount) % state.matchCount
+          : 0,
+    }));
+  },
+
+  clear: () => {
+    searchStore.set(initialState);
+  },
+};
+
+export const isSearchActive = derived(searchStore, ($search) => $search.isActive);
+
+export const searchQuery = derived(searchStore, ($search) => $search.query);