refactor: fix homepage file upload path (#5738)

Extracts file-based navigation logic from HomePage into pure function
with comprehensive test coverage.

New behavior:
- Opening 1 file from empty → switch to viewer (activeFileIndex: 0)
- Opening 2+ files from empty → switch to fileEditor
- pdfTextEditor tool → no auto-navigation (handles own empty state)
- Non-startup transitions (N→M files) → no navigation

Benefits:
- Pure function → easy to test and reason about
- Clear separation of concerns
- Preserves all existing behavior including pdfTextEditor special case
- Adds new multi-file startup behavior

Changes:
- HomePage.tsx: use getStartupNavigationAction() utility
- homePageNavigation.ts: pure navigation logic
- homePageNavigation.test.ts: comprehensive unit tests

Note: prevFileCountRef initialization kept as useRef(activeFiles.length)
to correctly handle files restored from IndexedDB on app startup.

# Description of Changes

<!--
Please provide a summary of the changes, including:

- What was changed
- Why the change was made
- Any challenges encountered

Closes #(issue_number)
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---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

frontend/src/core/pages/HomePage.tsx

@@ -22,6 +22,7 @@ import FileManager from "@app/components/FileManager";
 import LocalIcon from "@app/components/shared/LocalIcon";
 import { useFilesModalContext } from "@app/contexts/FilesModalContext";
 import AppConfigModal from "@app/components/shared/AppConfigModal";
+import { getStartupNavigationAction } from "@app/utils/homePageNavigation";
 
 import "@app/pages/HomePage.css";
 
@@ -60,22 +61,37 @@ export default function HomePage() {
   const { setActiveFileIndex } = useViewer();
   const prevFileCountRef = useRef(activeFiles.length);
 
-  // Auto-switch to viewer when going from 0 to 1 file
-  // Skip this if PDF Text Editor is active - it handles its own empty state
+  // Startup/open transition behavior:
+  // - opening exactly 1 file from empty -> viewer (unless already in fileEditor)
+  // - opening 2+ files from empty -> fileEditor
   useEffect(() => {
     const prevCount = prevFileCountRef.current;
     const currentCount = activeFiles.length;
 
-    if (
-      navigationState.workbench !== 'fileEditor' &&
-      prevCount === 0 &&
-      currentCount === 1
-    ) {
-      // PDF Text Editor handles its own empty state with a dropzone
-      if (selectedToolKey !== 'pdfTextEditor') {
-        actions.setWorkbench('viewer');
-        setActiveFileIndex(0);
+    console.log('[HomePage] Navigation effect triggered:', {
+      prevCount,
+      currentCount,
+      currentWorkbench: navigationState.workbench,
+      selectedToolKey,
+    });
+
+    const action = getStartupNavigationAction(
+      prevCount,
+      currentCount,
+      selectedToolKey,
+      navigationState.workbench
+    );
+
+    console.log('[HomePage] Navigation action returned:', action);
+
+    if (action) {
+      console.log('[HomePage] Applying navigation:', action);
+      actions.setWorkbench(action.workbench);
+      if (typeof action.activeFileIndex === 'number') {
+        setActiveFileIndex(action.activeFileIndex);
       }
+    } else {
+      console.log('[HomePage] No navigation - staying in current workbench');
     }
 
     prevFileCountRef.current = currentCount;