test(e2e): add Files page E2E test coverage

[patchmemory]

Summary

Adds 11 comprehensive E2E tests for the Files page that were salvaged from the stale task/task-core-architecture/mvp/selective-scan-cache branch (127 commits behind main).

What This PR Adds

New test file: tests/test_files_page_e2e.py

Test Coverage (11 tests)

1. ✅ Page loads successfully
2. ✅ Scan form has selection features (checkboxes, override controls)
3. ✅ Browse and scan integration workflow
4. ✅ Scan history unified display
5. ✅ Rclone scan with provider options
6. ✅ Snapshot browser after scan completion
7. ✅ Both /api/scan and /api/tasks available in UI
8. ✅ Provider panel displays file/folder details
9. ✅ Scan button integration with background form
10. ✅ Files page has all key sections
11. ✅ Provider selector and roots load correctly

Test Results

All 11 tests pass on current main:

============================= test session starts ==============================
tests/test_files_page_e2e.py::test_files_page_loads_successfully PASSED  [  9%]
tests/test_files_page_e2e.py::test_scan_form_has_selection_features PASSED [ 18%]
tests/test_files_page_e2e.py::test_browse_and_scan_integration PASSED    [ 27%]
tests/test_files_page_e2e.py::test_scan_history_unified_display PASSED   [ 36%]
tests/test_files_page_e2e.py::test_rclone_scan_with_options PASSED       [ 45%]
tests/test_files_page_e2e.py::test_snapshot_browser_after_scan PASSED    [ 54%]
tests/test_files_page_e2e.py::test_scan_and_tasks_apis_both_available PASSED [ 63%]
tests/test_files_page_e2e.py::test_provider_panel_displays_details PASSED [ 72%]
tests/test_files_page_e2e.py::test_scan_button_integration_with_background_form PASSED [ 81%]
tests/test_files_page_e2e.py::test_files_page_has_key_sections PASSED    [ 90%]
tests/test_files_page_e2e.py::test_provider_selector_and_roots_load PASSED [100%]

============================== 11 passed in 2.91s

What Happened to the Original Branch?

The branch task/task-core-architecture/mvp/selective-scan-cache was 127 commits behind main because:

• Main independently implemented selective scan cache and Files page selection features
• The branch work was redundant with what had already been merged
• This E2E test file was the only unique, valuable contribution from that branch

Notes

• Pre-existing test failure in test_folder_config_precedence exists on main (unrelated to this PR)
• Tests validate current main behavior (selection features, dual scan mechanisms)
• No changes to production code, only test coverage additions

🤖 Generated with Claude Code