fix: only plugins with on_install are managed - skip others entirely

- Discover(): skip plugins without on_install hook (not shown in UI)
- Enable(): simplified - just check Installed flag
- SyncConfig(): simplified - no HasInstall special case
- Frontend: simplified toggle logic (only installed/uninstalled states)
- Tests: updated all test fixtures to include on_install hook

frontend/src/lib/SettingsPlugins.svelte

@@ -107,11 +107,11 @@
             </div>
           </div>
           <div class="plugin-toggle">
-            {#if p.hasInstall && !p.installed}
+            {#if !p.installed}
               <button class="install-btn" on:click={() => install(p)}>
                 📦 Установить
               </button>
-            {:else if p.hasInstall && p.installed}
+            {:else}
               <div class="toggle-group">
                 <button
                   class="toggle-btn"
@@ -126,16 +126,6 @@
                   🗑
                 </button>
               </div>
-            {:else}
-              <button
-                class="toggle-btn"
-                class:active={p.active}
-                on:click={() => toggle(p)}
-                role="switch"
-                aria-checked={p.active}
-              >
-                <span class="toggle-knob"></span>
-              </button>
             {/if}
           </div>
         </div>