Replace switch-practice point-scan emulator with simple practice blocks #836

Problem

Switch testing currently embeds a sandboxed point-scan emulator (line/grid/menu Workflow in a 1280×720 box). Testing should be a simple set of blocks scanned among, not a miniature desktop point scan.

Acceptance criteria

• Practice shows a fixed set of labeled blocks (about 6), not derived from saved line/grid settings.
• Real local/remote switch gestures drive Select / Next / Previous / Stop via existing gesture mapping.
• Select starts scanning among blocks; Next/Previous move the highlight; Select chooses a block with feedback only (no native click/type).
• Idle Next/Previous explain that Select starts a scan (not Tab).
• Identify which binding fired and its press/hold action.
• Preserve isolation: pause live point scan, own capture, stop on Escape/Stop/emergency hold/focus loss/disconnect/timeout, cancel remote forwarding on end.
• Remove Workflow / point_scan engine usage from switch practice.
• Frontend shows a block grid instead of the SVG scan emulator; copy no longer claims practice uses saved line/grid settings.

Scope

Redesign of the practice path shipped in #834 / #826. Do not change live desktop point scanning.