fix: cancel click connections on pane click - #6068

Fixes #5237.

Clicking the empty pane cancels a pending click-to-connect operation in both React Flow and Svelte Flow. The click-connect-end callback receives the final connection state before the start handle is cleared; normal pane clicks, panning, selection drags, node clicks and handle-to-handle connections keep their existing behavior.

The React and Svelte fixtures now run through one framework-agnostic click-connect.spec.ts, using the configured framework's pane, viewport, selection, node, connection and edge selectors. The duplicate Svelte spec is removed. The patch changeset covers both packages.

Validation on Linux, at this revision:

• pnpm build and pnpm typecheck pass.
• Prettier passes for the shared spec.
• All seven cases pass for each framework on Chromium, Firefox and WebKit: 42 focused browser tests, with retries disabled.

From the repository root:

pnpm build
pnpm typecheck
pnpm exec prettier --check tests/playwright/e2e/click-connect.spec.ts
cd tests/playwright
FRAMEWORK=react pnpm exec playwright test e2e/click-connect.spec.ts -c playwright.react.config.ts --workers=2 --retries=0
FRAMEWORK=svelte pnpm exec playwright test e2e/click-connect.spec.ts -c playwright.svelte.config.ts --workers=2 --retries=0

This revision reruns the focused browser suite. The earlier full-suite results are historical: at 7abb6f0, React had 197 passing/25 skipped and Svelte had 187 passing/28 skipped/7 failures (six background-route 404s reproduced without the Svelte fix; the WebKit auto-pan case passed in an isolated rerun).