fix: workbench play/pause controls layout and split 2D into its own workspace

[Osayi-ANL]

Issue

Two related layout problems in the post-processing workbench:

1. Play/pause controls and inputs area is poorly laid out — btn_play, btn_pause, frame slider, and associated inputs are cramped or misaligned, making the playback controls hard to use.

2. 2D workspace is not its own independent tab — the 2D view (workspace_2d.py) is currently bundled alongside other workspace modes rather than having its own dedicated tab/panel. This makes it harder to work with 2D data independently.

Files:

• src/dashpva/viewer/workbench/workbench.py — play/pause wiring (btn_play, btn_pause at lines 1196–1221); workspace tab registration
• src/dashpva/viewer/workbench/workspace/workspace_2d.py — 2D workspace implementation; needs to register as a standalone tab
• src/dashpva/viewer/controls/controls_2d.py — 2D input controls; layout may need restructuring

Fixes

• src/dashpva/viewer/workbench/workbench.py — restructure the play/pause + frame input row into a clean horizontal layout with consistent spacing; ensure slider, play, pause, and frame number input are all visible and aligned
• src/dashpva/viewer/workbench/workspace/workspace_2d.py — register Workspace2D as its own independent tab in the workspace tab widget, separate from 3D and 1D modes
• src/dashpva/viewer/controls/controls_2d.py — adjust control layout to fit cleanly in the new standalone 2D tab

Acceptance Criteria

• Open the post workbench — play, pause, frame slider, and frame number input are aligned in a single uncluttered row
• The workspace tab bar has a dedicated "2D" tab that opens the 2D view independently
• Switching to the 2D tab does not affect the state of 1D or 3D workspaces
• Play/pause controls work correctly from within the 2D workspace tab

[Osayi-ANL]

Note: #97 and #96 are interdependent and are being delivered together on branch issue/96.

• fix: workbench play/pause controls layout and split 2D into its own workspace #97 splits the 2D viewer out of workbench.py into Workspace2D(BaseTab). The per-frame annotation work in feat: per-frame annotations (notes) in live view and post-processing workbench #96 (note label beneath the image, per-frame overlay refresh, edit hooks) is wired inside that Workspace2D, so feat: per-frame annotations (notes) in live view and post-processing workbench #96 builds directly on fix: workbench play/pause controls layout and split 2D into its own workspace #97's split.
• Both change the same regions of workbench.py, so they can't be reviewed or merged independently without conflicts.

They should be treated as a single unit (one PR) rather than merged separately.