Unify call-sign input fields (AutoCompleteBox vs ComboBox split) into a reusable component

[DeepDiver1975]

Problem

The ETB Von/An fields render visibly differently from the "Funkrufname" (call sign) field in Kräfte, Funktionen, and Atemschutz — reported as "the callsign autocomplete boxes on ETB look different than in other places."

Root cause

Not a styling bug — the app has two genuinely different control types in use for call-sign-ish fields, and they've never been reconciled:

AutoCompleteBox (free-text entry with suggestions, no dropdown arrow) — used for fields whose docstring reasoning is "must stay enterable, not a closed set":

• ForcesView.axaml — Brigade (BrigadeBox)
• RolesView.axaml — Name (PersonNameBox)
• OperatorPromptView.axaml — the operator's own call sign (CallSignBox)
• EtbView.axaml — Von/An (added in ETB #73)

ComboBox (closed picker, visible dropdown chevron) — used for the "Funkrufname" field specifically:

• RolesView.axaml:26
• ForcesView.axaml:38
• ScbaView.axaml:60

So 4 of 7 call-sign-ish fields across the app already use the free-text AutoCompleteBox pattern; the "Funkrufname" ComboBox in Roles/Forces/Scba is the outlier, not ETB.

Bonus bug found while investigating: RolesView.axaml:26 and ForcesView.axaml:38's Funkrufname ComboBox have no PlaceholderText set at all, so they render as a blank box with only an arrow — no label telling the user what it's for. ScbaView.axaml:60 does set PlaceholderText="Funkrufname" correctly.

Open decision (needs product input before implementing)

Which direction should "unify" go?

1. AutoCompleteBox everywhere — convert Roles/Forces/Scba's Funkrufname ComboBox to the same free-text-with-suggestions pattern as everywhere else. Matches the majority existing pattern and its stated reasoning (a unit/person's call sign might not be in Stammdaten yet either). Also fixes the missing-placeholder bug as a side effect. This is a real behavior change: users could type an arbitrary call sign for Kräfte/Funktionen/Atemschutz, not just pick from Stammdaten.
2. ComboBox everywhere — convert ETB's Von/An to a closed picker instead. This was deliberately rejected for ETB in ETB #73 (Von/An must accept non-Rufname senders like "ELW 1" or a person's name), so this direction would need re-litigating that decision.

Suggested next step

Once the direction is decided, extract a single reusable component (e.g. a small UserControl or a shared ControlTemplate/style) for "pick or type a call sign," used consistently by all five current call-sign fields (Roles, Forces, Scba, ETB Von, ETB An) plus the operator's own call-sign field, so future call-sign fields can't drift again the way this one did.