feat(aisidebar): implement Ollama availability handling and graceful startup

- Add comprehensive Ollama connection error handling strategy
- Implement OllamaClient with non-blocking initialization and connection checks
- Create OllamaAvailabilityMonitor for periodic Ollama connection tracking
- Update design and requirements to support graceful Ollama unavailability
- Add new project structure for AI sidebar module with initial implementation
- Enhance error handling to prevent application crashes when Ollama is not running
- Prepare for future improvements in AI sidebar interaction and resilience

.kiro/specs/ai-sidebar-enhancements/design.md

@@ -329,6 +329,31 @@ class PreferencesState:
 
 ## Error Handling
 
+### Ollama Unavailability
+
+- **Startup Without Ollama**: Initialize all components successfully, show status message in UI
+- **Model List Failure**: Return empty list, display "Ollama not running" in model label
+- **Chat Request Without Ollama**: Display friendly message: "Please start Ollama to use AI features"
+- **Connection Lost Mid-Stream**: Display partial response + reconnection instructions
+- **Periodic Availability Check**: Attempt to reconnect every 30s when unavailable (non-blocking)
+
+#### Implementation Strategy
+
+```python
+class OllamaClient:
+    def __init__(self, host: str | None = None) -> None:
+        # Never raise exceptions during initialization
+        # Set _available = False if connection fails
+        
+    def list_models(self) -> list[str]:
+        # Return empty list instead of raising on connection failure
+        # Log warning but don't crash
+        
+    def chat(self, ...) -> dict[str, str] | None:
+        # Return error message dict instead of raising
+        # {"role": "assistant", "content": "Ollama unavailable..."}
+```
+
 ### Streaming Errors
 
 - **Connection Lost**: Display partial response + error message, allow retry
@@ -422,6 +447,32 @@ class PreferencesState:
 - Preferences file is optional; defaults work without it
 - Graceful degradation if gtk4-layer-shell unavailable
 
+### Ollama Availability Detection
+
+Add periodic checking mechanism to detect when Ollama becomes available:
+
+```python
+class OllamaAvailabilityMonitor:
+    """Monitors Ollama availability and notifies UI of state changes."""
+    
+    def __init__(self, client: OllamaClient, callback: Callable[[bool], None]):
+        self._client = client
+        self._callback = callback
+        self._last_state = False
+        self._check_interval = 30  # seconds
+        
+    def start_monitoring(self) -> None:
+        """Begin periodic availability checks via GLib.timeout_add."""
+        
+    def _check_availability(self) -> bool:
+        """Check if Ollama is available and notify on state change."""
+```
+
+Integration in SidebarWindow:
+- Initialize monitor on startup
+- Update UI state when availability changes (enable/disable input, update status message)
+- Show notification when Ollama becomes available: "Ollama connected - AI features enabled"
+
 ### Future Enhancements
 
 - Command history with up/down arrow navigation