feat: auto-hide offline agents + fix server build errors

[wxfbot03-lang]

Summary

This PR contains two sets of changes:

1. New Feature: Auto-Hide Offline Agents (Client)

Adds an auto-hide offline agents toggle to the Sessions table view:

• UI Changes (AdaptixClient/):

  • SessionsTableWidget: Added checkAutoHideOffline checkbox that filters offline agents from the session list
  • DialogSettings: Added AutoHideOffline persistent setting
  • SessionsGraph: Graph view responds to the toggle by calling RebuildAll() to refresh the layout
  • Agent/Storage/Settings: Added AutoHideOffline field for persistence

• Behavior:

  • When enabled, offline agents are automatically hidden from the session list
  • Sessions Graph view updates accordingly
  • Setting is persisted to the database via Settings, preserved across restarts

2. Fix Server Build Errors

The dev-v1.3 branch references the TaskData.Priority field which was removed in axc2 v1.2.0, causing compilation failure.

• Fixed files (7 files total):

  • core/server/mgr_handler_job.go — Push(taskData.Priority, ...) → Push(0, ...)
  • core/server/mgr_handler_task.go — same as above
  • core/server/mgr_task.go — removed Priority validation logic (< 0 check)
  • core/server/ts_tasks.go — same as above
  • core/server/ts_tunnels.go — 2 occurrences, same as above
  • core/server/mgr_frame.go — removed unused logs import
  • extenders/beacon_agent/pl_main.go — removed Priority: priority from struct literal and deprecated PRIORITY_TASK assignment

• Dockerfile:

  • Build stage: golang:1.25-bookworm → golang:1.26-bookworm
  • Runtime stage: go1.25.4 → go1.26.4
  • Reason: go.work requires Go >= 1.26.1

Compatibility

• Qt version compatibility: checkStateChanged vs stateChanged (Qt >= 6.7.0 uses the former)
• All changes are backward compatible and do not affect existing functionality

Build Verification

• Client: macOS arm64 + Qt 6.11.1 — builds successfully
• Server + Extenders: Docker (linux/arm64) — builds successfully
• Server Docker container verified running normally