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.

Cargo.lock

@@ -11029,6 +11029,7 @@ dependencies = [
  "uuid",
  "yaak",
  "yaak-api",
+ "yaak-commands",
  "yaak-common",
  "yaak-core",
  "yaak-crypto",
@@ -11108,6 +11109,24 @@ dependencies = [
  "zip",
 ]
 
+[[package]]
+name = "yaak-commands"
+version = "0.0.0"
+dependencies = [
+ "log 0.4.29",
+ "serde_json",
+ "tempfile",
+ "thiserror 2.0.17",
+ "tokio",
+ "yaak",
+ "yaak-core",
+ "yaak-crypto",
+ "yaak-models",
+ "yaak-plugins",
+ "yaak-rpc-schema",
+ "yaak-templates",
+]
+
 [[package]]
 name = "yaak-common"
 version = "0.1.0"
@@ -11439,6 +11458,7 @@ version = "0.1.0"
 dependencies = [
  "regex 1.11.1",
  "tauri",
+ "yaak-core",
 ]
 
 [[package]]