Fix variable shadowing bug in utilCombinedTags sort

[JaiswalShivang]

Fixes #11931
In [utilCombinedTags()], the sort callback on line 401 does var key = key
intending to capture the outer loop variable. Due to JavaScript's var hoisting,
this actually creates a new [key] that is undefined, shadowing the outer one.

This means tagCounts[key + '=' + val] always looks up "undefined=..." which
returns undefined, so the frequency comparison is skipped entirely. Tag values
are sorted alphabetically instead of by frequency when multiple entities are selected.

Changes

• Changed var to [let] in the for...in loop for proper block scoping
• Removed the broken var key = key; // capture line

Why this works

Array.sort() runs synchronously, so the callback executes immediately while
the loop variable still holds the correct value. With [let], each iteration
gets its own binding — no capture needed.

[matkoniecz]

On which use case you have tested this?

[JaiswalShivang]

Tested on the preview deployment by selecting multiple road segments
at an intersection in Berlin (mix of Primary and Secondary roads).

The multi-entity tag editor renders correctly — no errors or regressions.
Combined tag values display as expected when clicking "Multiple Values" dropdowns.

The bug itself is a classic JavaScript var hoisting issue. Inside the sort
callback, var key = key hoists the declaration, so both sides of the
assignment refer to the inner (undefined) variable — the outer loop
variable is never captured. You can confirm this in any JS console:

var x = 10;
(function() { var x = x; console.log(x); })(); // undefined