fix(plugin-manager): sync UI state with plugin lifecycle + sidebar click fix

Root cause fixes:
- Sidebar: handleSidebarItem used item.id instead of item.view for viewId.
  Platform Test sidebar item has id=verstak.platform-test.sidebar but
  view=verstak.platform-test.diagnostics. Click now dispatches correct viewId.
- PluginManager: EnablePlugin/DisablePlugin only wrote to plugins.json but
  never re-discovered plugins. UI showed stale state (no Enable button after
  Disable, no Disable after Enable). Now calls ReloadPlugins() + loadAll()
  after each toggle.
- PluginManager: loadAll() fired async loads (GetCapabilities etc) without
  awaiting — loading spinner disappeared before data was ready. Now awaits
  all via Promise.all.
- PluginCard: no loading feedback on Enable/Disable buttons. Added
  actionFeedback prop — buttons show '⟳ Enabling...' / '⟳ Disabling...'
  and are disabled during operation.
- PluginManager: no visible result after Reload/Enable/Disable. Added
  toast notifications (success/error/info) with auto-dismiss.
- Settings: openSettingsFromProps didn't handle missing panel — now shows
  visible error in modal.

frontend/src/lib/plugin-manager/PluginCard.svelte

@@ -60,8 +60,12 @@
     !capabilities.some(c => c.name === opt)
   );
 
+  export let actionFeedback = {}; // { [pluginId]: 'enabling' | 'disabling' | null }
+
   $: isDisabled = p.status === 'disabled' || !p.enabled;
   $: canToggle = p.status !== 'failed' && p.status !== 'incompatible' && p.status !== 'missing-required-capability' && p.status !== 'discovered';
+  $: isBusy = actionFeedback[pluginId] != null;
+  $: busyAction = actionFeedback[pluginId] || null;
 </script>
 
 <div class="plugin-card" class:disabled={isDisabled} class:failed={p.status === 'failed'}>
@@ -181,12 +185,12 @@
     {/if}
     {#if vaultOpen && canToggle}
       {#if isDisabled}
-        <button class="btn-enable" on:click={() => onEnable(m.id)} type="button">
-          ▶ Enable
+        <button class="btn-enable" on:click={() => onEnable(m.id)} type="button" disabled={isBusy}>
+          {#if busyAction === 'enabling'}⟳ Enabling...{:else}▶ Enable{/if}
         </button>
       {:else}
-        <button class="btn-disable" on:click={() => onDisable(m.id)} type="button">
-          ⏸ Disable
+        <button class="btn-disable" on:click={() => onDisable(m.id)} type="button" disabled={isBusy}>
+          {#if busyAction === 'disabling'}⟳ Disabling...{:else}⏸ Disable{/if}
         </button>
       {/if}
     {/if}