fix(exploring): buttons stuck disabled after filter + layer/groupbox change v2.9.41 - ALL BACKENDS

🐛 **Problem:**
- Zoom/Identify buttons remained disabled after:
  1. Applying geometric filter on layer A (any backend)
  2. Switching to layer B
  3. Switching between groupboxes (single → multiple → custom)
  4. Applying 2nd filter in multi-step filtering
- **Affects ALL backends**: PostgreSQL, Spatialite, OGR
- Buttons didn't update even when features were selected in new layer/groupbox

🔍 **Root Cause:**
- `_update_exploring_buttons_state()` only called in `_handle_exploring_features_result()`
- Not triggered during:
  - Layer change via `current_layer_changed()`
  - Widget reload after filtering in `filter_engine_task_completed()`
  - Groupbox switch to single_selection via `_configure_single_selection_groupbox()`
- Result: Buttons retained previous state, creating UX inconsistency

✅ **Solution:**
1. Call `_update_exploring_buttons_state()` after `_reload_exploration_widgets()` in `current_layer_changed()`
2. Call `_update_exploring_buttons_state()` after widget reload in `filter_engine_task_completed()`
3. Call `_update_exploring_buttons_state()` at end of `_configure_single_selection_groupbox()` (other groupboxes already had it)

📊 **Impact:**
- Buttons now always reflect current selection state
- Works for ALL backends (PostgreSQL, Spatialite, OGR)
- Fixes 3 distinct scenarios: layer change, filter completion, groupbox switch
- Low risk, high UX benefit

📁 **Files Changed:**
- filter_mate_dockwidget.py (lines ~7106, ~10313)
- filter_mate_app.py (line ~4237)
- CHANGELOG.md (added v2.9.41 entry with ALL BACKENDS note)
- metadata.txt (version bump)
- docs/FIX_EXPLORING_BUTTONS_SPATIALITE_LAYER_CHANGE_v2.9.41.md (full analysis - updated for all backends)
- docs/SUMMARY_FIX_v2.9.41.md (quick reference)
- tools/test_v2.9.41.sh (validation script)

🧪 **Testing:**
- Manual tests required: 3 scenarios × 3 backends
- Automated validation: PASSED ✅
- Syntax checks: PASSED ✅

See: docs/FIX_EXPLORING_BUTTONS_SPATIALITE_LAYER_CHANGE_v2.9.41.md
