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/utils/homePageNavigation.ts

@@ -0,0 +1,49 @@
+import type { WorkbenchType } from '@app/types/workbench';
+
+export type StartupWorkbench = 'viewer' | 'fileEditor';
+
+export interface StartupNavigationAction {
+  workbench: StartupWorkbench;
+  activeFileIndex?: number;
+}
+
+export function getStartupNavigationAction(
+  previousFileCount: number,
+  currentFileCount: number,
+  selectedToolKey: string | null,
+  currentWorkbench: WorkbenchType
+): StartupNavigationAction | null {
+  console.log('[homePageNavigation] Called with:', {
+    previousFileCount,
+    currentFileCount,
+    selectedToolKey,
+    currentWorkbench,
+  });
+
+  // pdfTextEditor handles its own empty state
+  if (selectedToolKey === 'pdfTextEditor') {
+    console.log('[homePageNavigation] pdfTextEditor detected, returning null');
+    return null;
+  }
+
+  // Only handle transitions from empty (0 files) to some files
+  if (previousFileCount !== 0) {
+    console.log('[homePageNavigation] Not a 0→N transition, returning null');
+    return null;
+  }
+
+  // 0→1: Go to viewer to view the single file
+  if (currentFileCount === 1) {
+    console.log('[homePageNavigation] 0→1 transition, returning viewer');
+    return { workbench: 'viewer', activeFileIndex: 0 };
+  }
+
+  // 0→N (N>1): Go to fileEditor to manage multiple files
+  if (currentFileCount > 1) {
+    console.log('[homePageNavigation] 0→N transition, returning fileEditor');
+    return { workbench: 'fileEditor' };
+  }
+
+  console.log('[homePageNavigation] Still at 0 files, returning null');
+  return null;
+}