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/components/config/ServerTab.tsx

@@ -154,7 +154,10 @@ export function ServerTab(): React.JSX.Element {
           </Field>
         </div>
         <div className={styles.fieldRow}>
-          <Field label="DB Purge Age (s)">
+          <Field
+            label="DB Purge Age (s)"
+            hint="Ban records older than this are removed from the fail2ban database."
+          >
             <Input
               type="number"
               value={effectiveDbPurgeAge}
@@ -163,7 +166,10 @@ export function ServerTab(): React.JSX.Element {
               }}
             />
           </Field>
-          <Field label="DB Max Matches">
+          <Field
+            label="DB Max Matches"
+            hint="Maximum number of log-line matches stored per ban record."
+          >
             <Input
               type="number"
               value={effectiveDbMaxMatches}