handlers: populate the reinforcement slot from FriendGet

[Seltraeh]

handlers: populate the reinforcement slot from FriendGet

Branch: split/08-friend
Base: dev
Merge position: 08 of 13

Part of the PR #28 split. Each PR branches from dev and contains only its
own changes, so this diff is exactly one subsystem. The set is designed to be
merged in numeric order; merging all 13 reproduces PR #28 byte for byte
(verified against tree 79a4e065).

Later PRs in the series touch Handlers.hpp, GmeControllerHandlers.cpp and
UserInfo.cpp too, so once earlier ones land this branch may need a rebase.
Those conflicts are always additions on both sides — keep both. Maintainer
edits are enabled, so feel free to push the rebase directly to this branch.

One file. Makes the squad-select reinforce slot functional.

What's included

FriendGet now emits both lists, because the picker reads from either depending on which UI path the binary takes:

• xZH6EIQ7 carrying ReinforcementInfo
• tojMy68W carrying FriendInfo

The placeholder entry is built from the player's active-deck leader, falling back to their highest-level unit.

Two things worth knowing

The out-of-range user_unit_id is deliberate. The placeholder uses a value that cannot match a real row, so post-battle cleanup can't delete one of the player's own units.

FriendInfo.element takes a uint32_t, unlike UserUnitInfo which takes a string for the same concept. The element is converted on the way out. This contextual overload has bitten us before.

Verification

Squad select shows the placeholder friend; the friend's unit appears in slot 6 in battle; the Social tab Friend Info dialog renders.