lts/5: backport #3860 so the shell's search dropdown can be named (9th WCAG finding)

## Summary

`mj-dropdown` has no way to be given an accessible name on `lts/5`. The input that provides it (`AriaLabel`) was added on `next` by #3860 and its follow-ups, which have not been backported.

## Why it matters now

#4303 backported the Explorer shell a11y work (#4121) to `lts/5` and cleared **eight** of the nine WCAG 2.1 A/AA findings raised against the shell and shared primitives. This is the ninth, and it was correctly excluded — on `lts/5` the `AriaLabel="Search in"` attribute would bind to an input that does not exist and be silently inert, so shipping it would have looked like a fix while doing nothing.

The consequence is concrete: **the Explorer shell's global search dropdown is unnamed on `lts/5`.** A screen-reader user reaches a combobox with no accessible name.

## Why it is blocking something

Sidecar pins 5.51.x, and its WCAG 2.1 AA claim (Sidecar #665) is what drove #4303. That claim is not complete while a shell control in the primary navigation path is unnamed. Whether it is a blocker or a documented exception is a call for whoever owns the conformance statement — but it should be an explicit call rather than an omission nobody noticed.

## What is needed

Backport #3860 (and whichever follow-ups the `AriaLabel` input actually depends on) to `lts/5`, then add `AriaLabel="Search in"` to `shell.component.html` — the single line #4303 deliberately dropped, and the only difference between the two branches' copies of that file.

Line rules apply: patch-only, no migrations, no metadata.

## Verification when it lands

The one-line delta in #4303 is the whole surface. Diffing the added lines of `shell.component.html` between `lts/5` and `next` should then come back empty, which is the check I used to confirm #4303 was otherwise faithful.

Follow-up to #4303. Source: #4121, #3860.

Review #3860 and its follow-ups to identify the AriaLabel dependencies, then compare the lts/5 and next copies of shell.component.html. Backport the required changes without migrations or metadata, add AriaLabel="Search in", and verify that the added lines match between both branches.

Written by the indexing model from the issue text.