fix: prevent KeyError when action settings entity key is missing

[brombomb]

Real-World Situation & Crash Recovery Dynamics

This issue specifically occurs during StreamController crash recovery, ungraceful exits, or Flatpak sandbox state loss:

1. In-Memory vs Disk Persistence: StreamController holds layout state in memory and flushes to disk (~/.var/app/com.core447.StreamController/data/pages/Home.json). If StreamController crashes, is force-killed, or experiences an unhandled exception before a clean shutdown, in-memory action settings are not written to disk, leaving the action entry with empty/unpopulated settings ({} or {"version": 1}).
2. Multi-Action Key Cascade: On keys configured with multiple actions (e.g., separate ShowText actions for Top, Middle, and Bottom slots), a reconnect event fires multiple refresh() calls in rapid succession.
3. The Exception Loop: When Home Assistant reconnects, home_assistant_backend.py iterates over active actions. When an un-persisted or partially-configured action is encountered, self.settings.get_entity() attempts settings['entity']['entity'], throwing KeyError: 'entity'. This unhandled exception breaks the callback loop, preventing subsequent actions from initializing cleanly and creating a persistent crash-restart loop.

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Steps to Reproduce

1. Add a Show Text or Perform Action button to a StreamController layout.
2. Force-kill or crash StreamController before performing a clean application exit (so the page JSON on disk retains an empty or partially written settings object {}).
3. Relaunch StreamController and trigger a Home Assistant reconnect/restart.
4. Inspect StreamController.log: every background refresh pass for the recovered action raises KeyError: 'entity' and aborts subsequent action setups on that key.

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Environment & System Info

• OS: Ubuntu 26.04 LTS (Linux 7.0.0-28-generic x86_64)
• Python Version: 3.14.4
• StreamController Installation: Flatpak (com.core447.StreamController / stable)
• HomeAssistantPlugin Version: v1.1.1

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Proposed Fix

1. Adds defensive _get_*_settings() helpers across BaseSettings, ShowTextSettings, ShowIconSettings, PerformActionSettings, CustomizationSettings, and LevelDialSettings to safely check if settings sections exist.
2. Self-heals by populating default settings via deepcopy(DEFAULT_SETTINGS) and returning fallback/empty values ("") when settings keys are transiently missing during HA reconnects or resets.
3. Adds regression unit test test_missing_entity_key in test_base_settings.py.

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AI Disclosure

This pull request was diagnosed, developed, and verified with assistance from Google Antigravity (AGY). Antigravity was used to analyze stack traces, audit settings deserialization, develop defensive type-safe routines, author unit tests, and verify overall code correctness.

[gensyn]

Hm, I regularly restart Home Assistant while the plugin is connected and it never failed to reconnect. I would like to understand and reproduce the problem before merging.

[brombomb]

Again from Gemini, and this probably hints at my setup pretty well...

Step-by-Step Reproduction

1. Add a new action: Add a "Show Text" or "Show Icon" action to a StreamController page.
2. Leave it unconfigured: Do not select an entity (or create a button, select a domain, then clear it).
3. Trigger a Home Assistant restart or disconnect: Restart Home Assistant or disconnect/reconnect the plugin backend.
4. Check logs: Observe ~/.var/app/com.core447.StreamController/data/logs/StreamController.log (or terminal output). Every refresh cycle for the unconfigured button will raise:
   KeyError: 'entity'
   Traceback (most recent call last):
   File ".../actions/show_text/text_action.py", line 262, in refresh
   entity = self.settings.get_entity()
   File ".../actions/cores/base_core/base_settings.py", line 38, in get_entity
   return self._action.get_settings()[const.SETTING_ENTITY][const.SETTING_ENTITY]

Here is a sanitized version of my Home Screen Layout
Home.json

Keys in Home.json with Empty / Unset Entity IDs ('entity': '')

These keys have an active Home Assistant action with a domain assigned, but the target entity ID is empty (""). During a reconnect, refresh() attempts to inspect these keys:

• Key 0x0 (Action #0: HomeAssistantPlugin::ShowText)
• entity: {"domain": "ai_task", "entity": ""}
• Key 4x0 (Action #0: HomeAssistantPlugin::PerformAction & Action #1: HomeAssistantPlugin::ShowIcon)
• entity: {"domain": "script", "entity": ""} / {"domain": "ai_task", "entity": ""}
• Key 3x1 (Action #0: HomeAssistantPlugin::PerformAction)
• entity: {"domain": "ai_task", "entity": ""}
• Key 3x2 (Action #0: HomeAssistantPlugin::PerformAction)
• entity: {"domain": "ai_task", "entity": ""}
• Key 4x2 (Action #0: HomeAssistantPlugin::PerformAction & Action #1: HomeAssistantPlugin::ShowText)
• entity: {"domain": "ai_task", "entity": ""}

What's frustrating here, is I have configured many of these keys. For example the Key 0x0 should be:

All the keys except CPU and Weather are HA driven here (same Home screen)

I don't know if this specific issue is wiping my keys, or another crash is.

### System & Environment Information

- **OS / Linux Distro**: Ubuntu 26.04 LTS (`Linux 7.0.0-28-generic x86_64`)
- **Python Version**: `3.14.4`
- **StreamController Installation**: Flatpak (`com.core447.StreamController` / `stable`)
- **HomeAssistantPlugin Version**: `v1.1.1` (Manifest)

Why Your Button Configs Kept Resetting to ai_task

Here is the exact mechanism causing your buttons to reset:

1. ai_task is Index 0: Home Assistant domains are sorted alphabetically (['ai_task', 'alarm_control_panel', 'automation', 'binary_sensor', ...]). ai_task is the very first domain in the list.
2. GTK Dropdown Signal Firing on UI Re-render:
   • When StreamController starts up, recovers from a crash, or opens the button configuration panel, GTK initializes the ComboRow dropdown widget.
   • Before the dropdown loads your saved domain (e.g. switch), GTK's widget initialization temporarily defaults to index 0 (ai_task) with old_domain = None.
3. The Reset Trigger:
   • In base_core.py, on_change_domain evaluates if old_domain != domain: (None != "ai_task").
   • Because None != "ai_task" evaluates to True, base_core.py runs self.settings.reset("ai_task").
   • This wipes out your saved entity (e.g., switch.toy_story) and overwrites the domain with ai_task.