Add a Host trait and move DB/model commands off Tauri

Command handlers took Tauri types, so running them anywhere else meant
rewriting them. `yaak_commands::Host` is what a handler actually needs
from its surroundings: who the client is, what it's looking at, and the
shared managers. Handlers are generic over it and keep the router's
shape, so another host registers one with rpc_handler_async! and no
adapter at all.

28 commands move: models, deletes, response reads, encryption, plugin
info, export. ClientCtx implements Host, so the desktop adapters are one
line and behavior is unchanged.

PluginHost is separate because PluginManager is "spawn a Node sidecar
and talk to it" — a browser host runs plugins in a worker and can't hand
one back. Only 4 of the 28 need it; the rest work without a plugin
runtime, and the compiler says which is which.

Also drops yaak_core::AppContext, an earlier sketch of this that was
never implemented.

crates/yaak-core/src/context.rs

@@ -1,5 +1,3 @@
-use std::path::PathBuf;
-
 /// Context for a workspace operation.
 ///
 /// In Tauri, this is extracted from the WebviewWindow URL.
@@ -37,20 +35,3 @@ impl WorkspaceContext {
         self
     }
 }
-
-/// Application context trait for accessing app-level resources.
-///
-/// This abstracts over Tauri's `AppHandle` for path resolution and app identity.
-/// Implemented by Tauri's AppHandle and by CLI's own context struct.
-pub trait AppContext: Send + Sync + Clone {
-    /// Returns the path to the application data directory.
-    /// This is where the database and other persistent data are stored.
-    fn app_data_dir(&self) -> PathBuf;
-
-    /// Returns the application identifier (e.g., "app.yaak.desktop").
-    /// Used for keyring access and other platform-specific features.
-    fn app_identifier(&self) -> &str;
-
-    /// Returns true if running in development mode.
-    fn is_dev(&self) -> bool;
-}

crates/yaak-core/src/lib.rs

@@ -6,5 +6,5 @@
 mod context;
 mod error;
 
-pub use context::{AppContext, WorkspaceContext};
+pub use context::WorkspaceContext;
 pub use error::{Error, Result};