refactor(frontend): adopt the lifecycle and the notification surface

The remaining views, brought onto the two mechanisms added earlier in
this series.

The lifecycle: every cached primary view moves its document listeners,
intervals and event subscriptions off connect/disconnect and onto
`viewActivated`/`viewDeactivated`, so `autotag-view` stops fielding
keystrokes from Settings, `downloads-view`'s 30 s clock stops ticking
for the session, and an off-screen view stops rendering on every
search keystroke. `autotag-view` keeps a document listener only for
Escape, whose dialogs Phase 5 migrates to wa-dialog anyway.

The voice: the silent failures now speak — scan and full rescan (with
a guard against the double-click the coalescing window allowed), job
pause/resume/cancel, playlist delete, download request pause/remove/
clear, add and rename library, add-to-playlist, playlist track
removal, autotag's dialogs and its apply, and favourite reverts. Both
private toasts are gone, along with their CSS and keyframes. Playlist
delete (single and the multi-select loop), download-request removal,
download-client removal and a queue clear over 20 tracks ask first.

Loading, empty and failed become three states rather than one, in
`track-list` and `genre-details` — the first is on the first screen a
new user ever sees — and the Settings index panel seeds itself with
`GetIndexStatus()` instead of waiting forever for a change event.
`smart-playlist-editor` and `download-picker` take the request-version
guard `explore-view` already had.

`track-details` loads through one memoised dynamic import in all ten
openers, which is what takes its 42 kB out of the startup chunk: an
un-upgraded custom element is a real HTMLElement on which `?.show()`
throws, so each opener awaits it before touching the element its
template already rendered.

frontend/src/components/playlist-picker/playlist-picker.ts

@@ -14,6 +14,8 @@ import {
 import { Events } from '../../events';
 import type { playlist } from '@go/models';
 import '@components/duplicate-tracks-dialog/duplicate-tracks-dialog.js';
+import { notificationStore } from '@store/notification-store';
+import { describeError } from '@utils/describe-error';
 import type { DuplicateTracksDialog } from '@components/duplicate-tracks-dialog/duplicate-tracks-dialog.js';
 
 /**
@@ -188,6 +190,14 @@ export class PlaylistPicker extends LitElement {
             this.dispatchComplete();
         } catch (err) {
             console.error('Failed to add tracks to playlist:', err);
+            // Transient: the picker closes either way, and the tracks
+            // simply are not there — nothing to undo, only to retry
+            // (errors.m7).
+            notificationStore.transient({
+                key: 'playlist-add',
+                text: `Could not add ${this.filePaths.length === 1 ? 'that track' : `those ${this.filePaths.length} tracks`} to the playlist. ${describeError(err)}`,
+                detail: String(err),
+            });
         } finally {
             this.loading = false;
         }
@@ -223,6 +233,11 @@ export class PlaylistPicker extends LitElement {
             this.dispatchComplete();
         } catch (err) {
             console.error('Failed to create playlist:', err);
+            notificationStore.transient({
+                key: 'playlist-create',
+                text: `Could not create “${name}”. ${describeError(err)}`,
+                detail: String(err),
+            });
         } finally {
             this.loading = false;
         }