fix: space leader-key conflict in Excalidraw tabs (#309)

[junereycasuga]

Fixes the Space leader-key conflict in Excalidraw tabs (#309)

When Space is configured as the Vim leader key, ZenNotes currently consumes Space before Excalidraw can use it for its temporary hand/pan mode. This makes it impossible to hold Space and drag around the canvas.

This PR keeps the existing short-press Space leader behavior, while allowing a long Space press inside Excalidraw to pass through to Excalidraw for canvas panning.

Changes:

• Adds Excalidraw-specific Space handling in VimNav.
• Short Space press still opens leader mode.
• Long Space press activates Excalidraw hand/pan behavior.
• Pressing the next leader key before releasing Space still works as a leader sequence.
• Scopes the behavior to active Excalidraw editor surfaces.
• Leaves non-Space leader mappings unchanged.

Before:

After:

[junereycasuga]

Closing after commit 2b21343