feat(nav): full bookmarkable URL model — retire minimalist-URL + Share-view (E4 S2)

The query string is now the source of truth for the current view, so every URL is
bookmarkable/shareable. Reverses the early minimalist-URL rule (user's call).

- urlState: urlWithParams (read-modify-write delta on the live query, each provider
  owns its keys) + clearParams (strip only transient auth params) replace the whole-
  query stripUrlParams; shareUrl deleted; filterParamUpdates helper.
- NavigationProvider: page + ?chan mirrored to the address bar (pushState URL arg;
  history.state mechanics untouched); loadInitialPage resolves deep links
  (?v->plex, ?pl->playlists, filter/?chan->feed, bare->stored landing).
- FeedFiltersProvider: feed filters mirrored to the URL, gated on page===feed.
- useHistorySubview: optional SubviewUrl mapping; PlexBrowse mirrors the sub-view to
  ?v=<kind>:<args> (show/season/info/person/playlist; grid+player stay ephemeral).
- PlaylistsProvider: selected playlist mirrored to ?pl on the Playlists page.
- App: the 5 stripUrlParams calls become targeted clearParams (link/oauth/admin/
  access/verify/reset); the share-link hydrate effect removed.
- Retire the "Share view" feature: Sidebar + SavedViewsWidget buttons + i18n keys.

Browser tab title stays the short module/channel name (document.title untouched) —
the query never leaks into it, per the user's request.

Gate green: typecheck, knip, eslint (0 err), prettier, 247 vitest (urlState.dom.test
+9). No browser-E2E yet (most visual sprint — flagged for the epic-end review).

frontend/src/components/Sidebar.tsx

@@ -2,12 +2,9 @@ import { type ReactNode, useState } from "react";
 import { useTranslation } from "react-i18next";
 import { keepPreviousData, useQuery } from "@tanstack/react-query";
 import { qk } from "../lib/queryKeys";
-import { Library, Pencil, Share2, SlidersHorizontal, User } from "lucide-react";
+import { Library, Pencil, SlidersHorizontal, User } from "lucide-react";
 import { api, type FeedFilters, type Tag } from "../lib/api";
 import { defaultLayout, type PanelLayout } from "../lib/panelLayout";
-import { shareUrl } from "../lib/urlState";
-import { copyText } from "../lib/clipboard";
-import { notify } from "../lib/notifications";
 import { useDebounced } from "../lib/useDebounced";
 import { clearedFilters, useActiveFilters } from "../lib/useActiveFilters";
 import { applyDateChip, deriveDateKey } from "../lib/dateFilter";
@@ -126,14 +123,6 @@ export default function Sidebar({
   const activeCount = useActiveFilters(filters, setFilters).length;
   const active = activeCount > 0;
 
-  async function shareView() {
-    if (await copyText(shareUrl(filters))) {
-      notify({ level: "success", message: t("sidebar.shareCopied") });
-    } else {
-      notify({ level: "warning", message: t("sidebar.shareFailed") });
-    }
-  }
-
   function clearAll() {
     setCustomDates(false);
     setFilters(clearedFilters(filters));
@@ -321,14 +310,6 @@ export default function Sidebar({
       >
         {t("sidebar.clearAll")}
       </button>
-      <button
-        onClick={shareView}
-        title={t("sidebar.shareView")}
-        aria-label={t("sidebar.shareView")}
-        className="p-1.5 rounded-lg text-muted hover:text-fg hover:bg-card transition"
-      >
-        <Share2 className="w-4 h-4" />
-      </button>
     </>
   );