fix: Fix galaxy node clicking and navigation bugs

This commit fixes two critical bugs in the galaxy navigation:

**Bug #1: Direct node URLs redirected to /chat**
- Updated AppStateMachine to recognize /galaxy/* paths (including query params) as galaxy state
- Changed line 55 from `pathname === '/galaxy'` to `pathname === '/galaxy' || pathname.startsWith('/galaxy/')`
- Changed line 89 to compare pathname instead of lastNavigatedPathRef to preserve query params

**Bug #2: Modal closed when clicking nodes**
- Refactored ThoughtGalaxy to use URL query params (?node=xxx) instead of route params (/galaxy/node:xxx)
- This prevents component unmounting when switching between nodes
- Deleted app/galaxy/[node-id]/page.tsx (no longer needed)
- Updated app/galaxy/page.tsx with documentation comment
- Modified ThoughtGalaxy to:
  - Use useSearchParams() hook
  - Get selectedNodeId from query params
  - Update URL with query params on node click (doesn't cause remount)
  - Clear query params when modal closes

**Testing:**
- Verified manually with Playwright MCP that /galaxy?node=xxx preserves query params
- Verified state machine correctly recognizes galaxy state
- Created comprehensive Playwright test suite in tests/playwright/galaxy.spec.ts

**Files changed:**
- components/AppStateMachine.tsx: Fixed state machine to handle /galaxy/* paths and preserve query params
- components/ThoughtGalaxy.tsx: Refactored to use query params instead of route params
- app/galaxy/page.tsx: Added documentation
- app/galaxy/[node-id]/page.tsx: Deleted (replaced with query param approach)
- tests/playwright/galaxy.spec.ts: Added comprehensive test suite

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

components/AppStateMachine.tsx

@@ -52,7 +52,7 @@ export function AppStateMachineProvider({ children }: { children: React.ReactNod
       initialEvent = 'NAVIGATE_TO_CONVO';
     } else if (pathname === '/edit') {
       initialEvent = 'NAVIGATE_TO_EDIT';
-    } else if (pathname === '/galaxy') {
+    } else if (pathname === '/galaxy' || pathname.startsWith('/galaxy/')) {
       initialEvent = 'NAVIGATE_TO_GALAXY';
     }
 
@@ -84,8 +84,9 @@ export function AppStateMachineProvider({ children }: { children: React.ReactNod
       targetPath = '/galaxy';
     }
 
-    // ONLY navigate if we have a target path and haven't already navigated to it
-    if (targetPath && targetPath !== lastNavigatedPathRef.current) {
+    // ONLY navigate if we need to change the pathname (not the whole URL)
+    // This preserves query params and prevents unnecessary navigation
+    if (targetPath && pathname !== targetPath) {
       console.log('[App Provider] State machine navigating to:', targetPath);
       lastNavigatedPathRef.current = targetPath;
       router.push(targetPath);