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-commands/src/plugins.rs

@@ -0,0 +1,50 @@
+//! Plugin queries that only need the plugin manager and the database.
+
+use crate::error::Result;
+use crate::host::PluginHost;
+use std::path::PathBuf;
+use yaak_plugins::plugin_meta::{PluginMetadata, get_plugin_meta};
+use yaak_rpc_schema::*;
+
+pub async fn cmd_plugin_info<H: PluginHost>(
+    host: H,
+    req: CmdPluginInfoReq,
+) -> Result<PluginMetadata> {
+    let plugin = host.db().get_plugin(&req.id)?;
+    if let Some(plugin_handle) =
+        host.plugin_manager().get_plugin_by_dir(plugin.directory.as_str()).await
+    {
+        return Ok(plugin_handle.info());
+    }
+
+    if let Ok(metadata) = get_plugin_meta(&PathBuf::from(&plugin.directory)) {
+        return Ok(metadata);
+    }
+
+    Ok(fallback_plugin_metadata(&plugin.directory))
+}
+
+fn fallback_plugin_metadata(directory: &str) -> PluginMetadata {
+    let display_name = PathBuf::from(directory)
+        .file_name()
+        .and_then(|name| name.to_str())
+        .filter(|name| !name.is_empty())
+        .unwrap_or(directory)
+        .to_string();
+
+    PluginMetadata {
+        version: "Unavailable".to_string(),
+        name: directory.to_string(),
+        display_name,
+        description: Some(format!("Plugin metadata could not be loaded from {directory}")),
+        homepage_url: None,
+        repository_url: None,
+    }
+}
+
+pub async fn cmd_plugin_init_errors<H: PluginHost>(
+    host: H,
+    _req: CmdPluginInitErrorsReq,
+) -> Result<Vec<(String, String)>> {
+    Ok(host.plugin_manager().take_init_errors().await)
+}