feat(iplimit): gate IP limit on fail2ban and reset stale limits

Per-client IP limit only enforces where fail2ban is installed, so the panel now reports enforceability and disables the field otherwise:

- Add GET /panel/api/server/fail2banStatus (enabled/installed/usable/windows), cached 30s.
- ClientFormModal and ClientBulkAddModal disable the IP Limit input when not usable and show a hover tooltip; Windows gets a platform-specific message instead of the bash-menu hint.
- One-time migration ResetIpLimitNoFail2ban zeroes existing client limitIp (inbound settings JSON + clients table) on hosts without fail2ban, where the limit never applied.
- Drop the recurring '[LimitIP] Fail2Ban is not installed' warning.
- Add limitIpFail2banMissing/limitIpFail2banWindows/limitIpDisabled across all 13 locales.

frontend/src/api/queryKeys.ts

@@ -1,6 +1,7 @@
 export const keys = {
   server: {
     status: () => ['server', 'status'] as const,
+    fail2banStatus: () => ['server', 'fail2banStatus'] as const,
   },
   nodes: {
     root: () => ['nodes'] as const,