refactor: Unify pull and push mode architectures for standalone viewing

[maragall]

This refactor eliminates duplicate T/FOV sliders when using load_dataset() (pull mode) by adopting a hybrid architecture:

Problem

When loading datasets via load_dataset(), NDV's ArrayViewer created its own sliders for the 6D array dimensions (time, fov, z, channel, y, x), while LightweightViewer also had custom T/FOV sliders - resulting in duplicate controls.

Solution: Hybrid Architecture

• Pull mode (load_dataset): Build the full 6D dask array once, then use custom T/FOV sliders to navigate via NDV's display_model API (fast, no array rebuilds)
• Push mode (start_acquisition/register_image): Keep existing 4D array rebuild approach for live acquisition

Key Changes

New Internal State

• _pull_mode: Flag to track navigation mode (True = fast NDV navigation)
• _is_ome_format: Track dataset format for proper plane loading
• _ome_file_index: Map FOV indices to OME-TIFF filepaths
• _pixel_size_um, _dz_um: Scale metadata for 3D rendering

New Methods

• _navigate_ndv(dim, value): Navigate NDV viewer without array rebuilds
• _hide_ndv_dimension_sliders(dims): Hide NDV's built-in time/fov sliders
• _insert_sliders_into_ndv_layout(): Position custom sliders adjacent to NDV's z-slider for cohesive visual grouping
• _on_ndv_ndims_requested(ndims): Re-hide sliders when 2D/3D toggle clicked
• _build_6d_lazy_array(): Build complete 6D dask array for pull mode
• _load_ome_plane(): Load planes from OME-TIFF format files
• _scan_ome_tiff_to_state(): Scan OME-TIFF datasets

Modified Slider Handlers

• _on_time_slider_changed: Check _pull_mode to either navigate NDV or rebuild array
• _on_fov_slider_changed: Same hybrid behavior

UI Improvements

• Custom T/FOV sliders now appear directly below NDV's z-slider
• NDV's duplicate time/fov sliders are hidden via official API
• Signal connection prevents slider duplication when toggling 3D mode

Testing

Tested with 9 datasets covering single-TIFF and OME-TIFF formats:

• 20x_scan, DIPG17pons, empty_slide_20x, fcs2_20x_left, Monkey, ps-m1-t72, successful_run, test_10x_laser_af_z_stack, tissue_slide_20x