feat(headers): add optional dropdown to pick header names and values

[WkdXeqtr]

Summary

I'm migrating from Advanced REST Client (ARC), where header fields had a dropdown
of common names and values, so you could pick one without knowing the exact string
up front. Yaak already has inline autocomplete for headers - this PR adds an
optional dropdown on top of it, so the same suggestions are discoverable by
clicking, not only by typing.

It is fully opt-in and off by default
(Settings -> Interface -> Editor -> "Header suggestion dropdowns"), so nothing
changes unless you turn it on. When enabled, a small chevron appears on the header
name and value inputs; picking an item inserts it. Inline autocomplete is left
untouched.

The dropdown reuses Yaak's existing header suggestion data (header names, mime
types, encodings, ...) and adds a few common value lists (Cache-Control,
Accept-Language, User-Agent). For values that are long or hard to remember, like
User-Agent, it shows a short label (e.g. "Chrome 120 - Windows") but inserts the
full string, using the completion's apply field so inline autocomplete shows the
same labels.

Submission

• This PR is a bug fix or small-scope improvement.
• If this PR is not a bug fix or small-scope improvement, I linked an approved feedback item below.
• I have read and followed CONTRIBUTING.md.
• I tested this change locally.
• I added or updated tests when reasonable.

Approved feedback item (required if not a bug fix or small-scope improvement):

Related

[gschier]

JUst added this to the PR template (for future PRs), but can you please attach screenshots or a screen recording of this? I would also consider it a feature, not a small-scoped improvement, but I'm willing to consider it if it fits the vibe of Yaak.

[WkdXeqtr]

Thanks for taking a look! Screenshots below.

The setting (off by default, Settings -> Interface -> Editor):

A header field with it off (unchanged, no chevron) vs on:

Picking a value from the dropdown:

[gschier]

Thanks for that!

I wasn't sure I'd like this at first but, now that I see it, I think I do :)

Going to review and get this in for the next release

[WkdXeqtr]

Hey, just checking in on this one. I updated to 2026.5.0 and didn't see the dropdowns there, so I guess it didn't make it into the release.

I see a lot has landed on main since July, so I'm probably just in the way of it now. Anything you'd like me to do on my side - rebase, split it up, change something?

No rush either way, just didn't want it to get lost.

[gschier]

Ah ya, sorry about that. Added a Feedback item for it (I use this as my to-do list) so I won't forget:

https://yaak.app/feedback/posts/optional-dropdown-to-pick-header-names-and-values