feat(owlry-core): move backend modules from owlry

Move the following modules from crates/owlry/src/ to crates/owlry-core/src/:
- config/ (configuration loading and types)
- data/ (frecency store)
- filter.rs (provider filtering and prefix parsing)
- notify.rs (desktop notifications)
- paths.rs (XDG path handling)
- plugins/ (plugin system: native loader, manifest, registry, runtime loader, Lua API)
- providers/ (provider trait, manager, application, command, native_provider, lua_provider)

Notable changes from the original:
- providers/mod.rs: ProviderManager constructor changed from with_native_plugins()
  to new(core_providers, native_providers) to decouple from DmenuProvider
  (which stays in owlry as a UI concern)
- plugins/mod.rs: commands module removed (stays in owlry as CLI concern)
- Added thiserror and tempfile dependencies to owlry-core Cargo.toml

crates/owlry-core/src/plugins/error.rs

@@ -0,0 +1,51 @@
+//! Plugin system error types
+
+use thiserror::Error;
+
+/// Errors that can occur in the plugin system
+#[derive(Error, Debug)]
+#[allow(dead_code)] // Some variants are for future use
+pub enum PluginError {
+    #[error("Plugin '{0}' not found")]
+    NotFound(String),
+
+    #[error("Invalid plugin manifest in '{plugin}': {message}")]
+    InvalidManifest { plugin: String, message: String },
+
+    #[error("Plugin '{plugin}' requires owlry {required}, but current version is {current}")]
+    VersionMismatch {
+        plugin: String,
+        required: String,
+        current: String,
+    },
+
+    #[error("Lua error in plugin '{plugin}': {message}")]
+    LuaError { plugin: String, message: String },
+
+    #[error("Plugin '{plugin}' timed out after {timeout_ms}ms")]
+    Timeout { plugin: String, timeout_ms: u64 },
+
+    #[error("Plugin '{plugin}' attempted forbidden operation: {operation}")]
+    SandboxViolation { plugin: String, operation: String },
+
+    #[error("Plugin '{0}' is already loaded")]
+    AlreadyLoaded(String),
+
+    #[error("Plugin '{0}' is disabled")]
+    Disabled(String),
+
+    #[error("Failed to load native plugin: {0}")]
+    LoadError(String),
+
+    #[error("IO error: {0}")]
+    Io(#[from] std::io::Error),
+
+    #[error("TOML parsing error: {0}")]
+    TomlParse(#[from] toml::de::Error),
+
+    #[error("JSON error: {0}")]
+    Json(#[from] serde_json::Error),
+}
+
+/// Result type for plugin operations
+pub type PluginResult<T> = Result<T, PluginError>;