fix(share): stop ShareDialog effect from wedging the scheduler in prod (#686)

ShareDialog tracked the open transition with `let prevOpen = $state(false)`
and an `$effect.pre` that both read and wrote `prevOpen` — a self-invalidating
effect. In dev this trips `effect_update_depth_exceeded`; in a production
build the guard differs and it silently wedges Svelte's global effect
scheduler the moment `open` flips on close. Result: after opening+closing the
share dialog, effects stop flushing app-wide — clicks change the URL but the
view never re-renders (no error, no CPU spin).

Make `prevOpen` a plain variable (it's only used for edge-detection inside the
effect), so the effect depends solely on `open` and never re-triggers itself.

web/src/lib/components/ShareDialog.svelte

@@ -33,8 +33,12 @@
 	let newlyCreatedLinkId = $state<string | null>(null);
 	let deletingLinkId = $state<string | null>(null);
 
-	// Track previous open state to detect open transitions
-	let prevOpen = $state(false);
+	// Track previous open state to detect open transitions. This is a PLAIN
+	// variable (not $state) on purpose: it's only read/written inside the
+	// effect below for edge-detection. Making it $state turned the effect
+	// into a self-invalidating loop (it writes a state it reads), which in a
+	// production build silently wedges Svelte's effect scheduler on close.
+	let prevOpen = false;
 
 	$effect.pre(() => {
 		if (open && !prevOpen) {