Add uniform save-failure UX (toast/log) for useSaving.runSave onError paths

[coderabbitai]

Summary

During review of PR #5273 (save-consolidation onto useSaving.runSave / useReboot), it was noted that useSaving's runSave (and every migrated onError callback across the 10 migrated tabs) only logs failures via console.error. There is no user-facing toast/notification when a save fails.

Rationale

Users currently get no visible feedback if a configuration save fails (e.g. MSP write rejection, timeout, etc.), beyond a browser console error. A uniform "save failed" UX would improve error visibility across all save flows.

Affected areas

• src/composables/useSaving.js (runSave default error handling)
• All tabs/composables migrated to runSave in PR refactor(save): consolidate all config saves onto one runSave + saveToEeprom path #5273, including:
  • src/components/tabs/PowerTab.vue
  • src/components/tabs/GpsTab.vue
  • src/components/tabs/OnboardLoggingTab.vue
  • src/components/tabs/ServosTab.vue
  • src/components/tabs/AuxiliaryTab.vue
  • src/composables/adjustments/useAdjustmentsSave.js
  • src/components/tabs/LedStripTab.vue
  • src/components/tabs/VtxTab.vue
  • src/components/tabs/PidTuningTab.vue
  • src/components/tabs/MotorsTab.vue

Proposed change

Introduce a consistent failure notification (e.g. via gui_log) either as a default behavior inside useSaving's runSave when no onError is provided, or as a shared helper that all onError callbacks invoke, so users see a clear "save failed" message instead of only a console error.

Acceptance criteria

• A failed save surfaces a user-visible error message (toast/log) consistently across all tabs using runSave.
• Behavior is centralized (not duplicated per-tab) to avoid drift.

References

• PR: refactor(save): consolidate all config saves onto one runSave + saveToEeprom path #5273
• Discussion: refactor(save): consolidate all config saves onto one runSave + saveToEeprom path #5273 (comment) (see review thread on ServosTab onError)
• Requested by: @haslinghuis