fix(export): stop the filename field from stealing the cursor while typing

[TobbyTravel]

Summary

• Fixes [Bug]: export filename field constantly moves cursor to end of line #1071 — the Export tab's Filename field kept resetting the caret to the end of the line roughly once a second, making it nearly impossible to type into.
• Root cause: ExportSettingsForm.load() runs on every debounced AppState resync (config_updated -> 150ms timer in right_panel.py). Typing in the Filename field routes through the export sidebar's own 500ms persist debounce (update_timer in export.py), which writes the value back to AppState and fires config_updated, which calls load() again. QLineEdit.setText() unconditionally moves the caret to the end, even when the text is unchanged, so any pause of ~650ms between keystrokes yanks the cursor to the end.
• Fix: skip setText() on the destination text fields (filename_edit, subfolder_edit, abspath_edit) while they have focus — the field's own textChanged handler already keeps AppState current, so there's nothing to restore.

Test plan

• Added test_load_does_not_reset_cursor_in_focused_filename_field, which reproduces the bug by focusing the field, positioning the caret mid-string, then calling load() again as the debounced resync would — fails before the fix (caret jumps to the end), passes after.
• uv run pytest tests/test_export_settings_form.py -v — 27 passed
• uv run pytest tests/ -q — full suite passes (5437 passed, 26 skipped, 14 deselected)
• make format / ruff check — clean
• ty check on the changed file — no new diagnostics (one pre-existing unrelated warning at line 536)

[TobbyTravel]

Code review caught that `cs_output_path_edit` (Contact Sheet → Output path) goes through the identical `config_updated` → 150ms `_sync_debounce` → `sync_ui()` round-trip as the Filename field, and was still calling `setText()` unconditionally — same bug, same field class. Pushed a follow-up commit applying the same focus guard there, plus a regression test (`test_sync_ui_does_not_reset_cursor_in_focused_contact_sheet_path`) that fails without the fix and passes with it.

I checked the rest of `export.py`/`export_settings_form.py` for other unconditional `setText()` calls on user-typed fields — the remaining ones (`cs_output_path_edit` from the folder-picker, `abspath_edit` from the folder-picker, and various label/button text) are all direct responses to a user action, not periodic resyncs, so they're fine as-is.