hotfix: plugin manager infinite loading

- frontend: RPC timeout (8s), try/catch/finally on reload, proper UI
  states (loading/error/empty/list)
- frontend: reload() now sets loading=true, catches errors
- backend: tilde expansion (~/.config/verstak/plugins → /home/mirivlad/...)
- backend: ReloadPlugins returns diagnostics (count, summary string)
- backend: diagnostic logging in DiscoverPlugins (start/dirs/entries/results)
- backend: FormatDiscoverySummary helper
- testing: 11 headless tests for DiscoverPlugins (empty, missing, valid,
  broken JSON, duplicate ID, multiple dirs, nonexistent mix)

frontend/wailsjs/go/api/App.d.ts

@@ -13,6 +13,6 @@ export function GetPermissions():Promise<Array<permissions.Entry>>;
 
 export function GetPlugins():Promise<Array<plugin.Plugin>>;
 
-export function ReloadPlugins():Promise<void>;
+export function ReloadPlugins():Promise<number|string>;
 
 export function Startup():Promise<void>;