Fix "no such file" error creating clients on non-default servers

SuggestIPAllocation always looked up the legacy "wg0" server regardless
of which server the request was for, so any setup without a migrated
wg0 (i.e. every fresh multi-server install) failed with
"open db/servers/wg0.json: no such file or directory" when adding a
new client.

Now accepts an optional server_id query param (falling back to wg0 for
the legacy bare routes), and the per-server clients page passes its
own server ID.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

DEVLOG.md

@@ -2169,3 +2169,36 @@ Keine Commits in dieser Session.
 - util/util.go                     |  63 +++++++++++++++++++++-----
 
 ---
+## 2026-07-12 23:27 – 23:27 (0m)
+**Beschreibung:** Claude Code Session
+**Projekt:** wireguard-ui-multi
+
+### Commits
+- 8a92958 Update DEVLOG session log
+
+### Geänderte Dateien
+- DEVLOG.md | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
+
+---
+## 2026-07-12 23:29 – 23:30 (1m)
+**Beschreibung:** Claude Code Session
+**Projekt:** wireguard-ui-multi
+
+### Commits
+Keine Commits in dieser Session.
+
+### Geänderte Dateien
+- DEVLOG.md | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
+
+---
+## 2026-07-12 23:32 – 23:34 (2m)
+**Beschreibung:** Claude Code Session
+**Projekt:** wireguard-ui-multi
+
+### Commits
+Keine Commits in dieser Session.
+
+### Geänderte Dateien
+- DEVLOG.md | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
+
+---