Target Selection: fully disable buttons during placement; lock selection + disable Add/Import during Edit

[peterhollender]

Two UX issues on Target Selection from smoke-testing SlicerOpenLIFU#643.

1. Placement mode leaves Edit / Remove clickable

Currently refresh_action_buttons() uses:

placing = self.placement_node is not None
...
self.add_button.enabled = loaded and not placing
self.import_button.enabled = loaded and not placing
self.edit_button.enabled = loaded          # <- still enabled during placement
self.remove_button.enabled = loaded and has_selection  # <- also still enabled

Add Target and Import correctly disable while the user is picking a slice-view location, but Edit and Remove don't -- clicking either mid-placement leaves the placement in a weird half-alive state.

Fix: placing should disable all four buttons. The only way out of placement mode is Escape (which fires EndPlacementEvent with 0 control points and cleans up).

2. Edit mode leaves Add / Import enabled and row selection free

Once the page is in Edit mode focused on one target (either because the user clicked Edit, or because they just placed a new one and were auto-dropped into Edit mode), the user can:

• Still click Add Target -- starts a new placement while another edit is in progress.
• Still click Import -- same problem.
• Click other rows in the table -- switches Edit focus away from the currently-locked-open target.

Each of those breaks the "you are editing ONE target at a time" contract that the fiducial lock model relies on.

Fix:

• While in Edit mode (is_in_edit_mode == True), disable Add Target and Import. The user must click Done first.
• Lock the table row selection to the row that Edit mode focused on. Programmatic + Done-click selection changes are OK; user click on another row silently reverts. Simplest implementation: track edit_focus_row at Edit toggle time, and on selection-change while in edit mode, if the new row differs, revert with a SignalBlocker and re-apply edit focus.
• Edit button itself stays enabled (says "Done") so the user can exit.
• Remove stays enabled during Edit mode -- keeps the current confirmation-dialog behaviour. (The user considered turning Remove into a no-confirm "Cancel" for freshly-placed targets specifically; they were fine deferring that if it adds branchy state. Skipping.)

Non-goals

• No change to placement lifecycle beyond disabling more buttons.
• No change to the confirmation dialog on Remove.
• No new "Cancel Placement" button. Escape mid-placement remains the sole cancel path.

Acceptance

1. Click Add Target in Target Selection. Add / Import / Edit / Remove are all disabled while the page waits for the slice-view click.
2. Press Escape → placement cancels; buttons return to normal (Add / Import / Edit enabled; Remove enabled if a row is selected).
3. Click Add Target → click a slice view. The new target is placed. Edit toggles ON, focused on the new row.
4. In this Edit mode, verify: Add and Import are disabled. Edit button says Done. Remove is enabled.
5. Click another row in the table. Selection reverts to the just-placed row; that row's fiducial stays the unlocked one.
6. Click Done. Edit exits, Add / Import / Edit return to enabled, Remove enabled iff a row is selected.
7. Select an existing target row, click Edit. Same lock behaviour: Add / Import disabled, selection locked to that row.

Relates to SlicerOpenLIFU#643, SlicerOpenLIFU#641, SlicerOpenLIFU#640.