Tasks search box layout on phone

On /tasks at 390px width the pane-header search field shows only about two characters of the typed term ("bc" for "board Live"), because the toolbar buttons (live, filter, templates, +, ▾) take the row. - Give the search field the full row on phone (buttons wrap below or collapse into one menu), so a typed term of ~20 characters is readable. - Keep the clear (✕) button inside the field without covering the text. Out of scope: search semantics and the server side (task 11567 shipped those); desktop layout. Done means: a 390px screenshot of /tasks?q=board%20Live shows the whole term in the field, with every toolbar action still reachable.

On phone the /tasks search box now takes the whole header line and the actions wrap under it, ranged right. The 390px visual check shows "board Live" whole, with the ✕ clear of it. Quick gate and changed-area tests green. PR #7427 queued to the merge lane. Memory: none applied.