Merge Global tab into Server tab and remove Global tab

Global tab provided the same four editable fields as Server tab:
log_level, log_target, db_purge_age, db_max_matches. Server tab already
has these fields plus additional read-only info (db_path, syslog_socket)
and a Flush Logs button.

- Add hint text to DB Purge Age and DB Max Matches fields in ServerTab
- Remove GlobalTab component import from ConfigPage
- Remove 'global' from TabValue type
- Remove Global tab element from TabList
- Remove conditional render for GlobalTab
- Remove GlobalTab from barrel export (index.ts)
- Delete GlobalTab.tsx file
- Update ConfigPage test to remove Global tab test case

All 123 frontend tests pass.

frontend/src/pages/ConfigPage.tsx

@@ -8,8 +8,7 @@
  *   Jails        — per-jail config accordion with inline editing
  *   Filters      — structured filter.d form editor
  *   Actions      — structured action.d form editor
- *   Global       — global fail2ban settings (log level, DB config)
- *   Server       — server-level settings + flush logs
+ *   Server       — server-level settings, logging, database config + flush logs
  *   Map          — map color threshold configuration
  *   Regex Tester — live pattern tester
  *   Export       — raw file editors for jail, filter, and action files
@@ -20,7 +19,6 @@ import { Tab, TabList, Text, makeStyles, tokens } from "@fluentui/react-componen
 import {
   ActionsTab,
   FiltersTab,
-  GlobalTab,
   JailsTab,
   LogTab,
   MapTab,
@@ -58,7 +56,6 @@ type TabValue =
   | "jails"
   | "filters"
   | "actions"
-  | "global"
   | "server"
   | "map"
   | "regex"
@@ -89,7 +86,6 @@ export function ConfigPage(): React.JSX.Element {
         <Tab value="jails">Jails</Tab>
         <Tab value="filters">Filters</Tab>
         <Tab value="actions">Actions</Tab>
-        <Tab value="global">Global</Tab>
         <Tab value="server">Server</Tab>
         <Tab value="map">Map</Tab>
         <Tab value="regex">Regex Tester</Tab>
@@ -100,7 +96,6 @@ export function ConfigPage(): React.JSX.Element {
         {tab === "jails" && <JailsTab />}
         {tab === "filters" && <FiltersTab />}
         {tab === "actions" && <ActionsTab />}
-        {tab === "global" && <GlobalTab />}
         {tab === "server" && <ServerTab />}
         {tab === "map" && <MapTab />}
         {tab === "regex" && <RegexTesterTab />}