fix: empty-canvas nudge stays gone once a tool is armed

[bvnaik05]

Summary

• Fixes Empty-canvas 'Nothing here yet' nudge reappears after arming and unarming a tool #564: the block/unified canvas "Nothing here yet" nudge only checked canvas emptiness, so it ignored tool-arming state and reappeared after arming a tool and then unarming back to select on an empty canvas.
• useEditorUi now latches state.hasArmedTool = true the first time any tool is armed (setTool to non-select, setDrawShape, armStarter, armComment) and never resets it.
• blockEmpty in DiagramCanvas.vue now returns false once hasArmedTool is set, so the nudge disappears immediately on first arm and stays gone for the rest of the session, even after unarming.

Test plan

• yarn test — all 150 files / 1743 tests pass, including new hasArmedTool cases in useEditorUi.test.js
• yarn build — succeeds
• Manually verified in the running app: nudge shows on a fresh empty diagram, disappears immediately on arming the Quadrilateral shape tool, and stays hidden after switching back to Select with the canvas still empty

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge.

The latch is consistently applied across tool-arming paths and the empty-state behavior matches the changed tests without an established failure.

_{Reviews (1): Last reviewed commit: "fix: empty-canvas nudge stays gone once ..." | Re-trigger Greptile}

[vibhavkatre]

Correct fix, and the latch is documented where it matters. Checked that createEditorUi() is per-page (EditorShell.vue:47, ViewerPage.vue:31), so hasArmedTool is scoped to one editor session and a freshly opened diagram still gets its nudge.

The product call the comment makes — a first-run nudge, not a persistent empty state, so it stays gone even after the canvas is emptied again — is the right one and is written down where the next reader will find it.

Merging.