feat(portals): fold live TV panels in nested levels with a category dropdown (#1556)

## Summary

Live TV panels now fold from the outside in, in three nested levels, instead of one toggle that hid the categories rail and the channel list together:

1. **Categories + channels + player** (browse, unchanged).
2. **Channels + player** — a new `chevron_left` in the categories rail header hides only that rail. The channels header then turns its title into a **category dropdown** that opens the same shell panel as a popover (search, sort, counts, selection are one implementation), plus a `chevron_right` that brings the rail back.
3. **Player only** — the channels header chevron, as before. The floating restore handle and `Cmd/Ctrl+B` return to the level the user collapsed from, not always to level 1.

Every level is restored as stored, per surface (`live-sidebar-state:<surface>`, from #1555): a hidden rail is discoverable through the workspace header toggle and the hidden-list empty state that #1555 added, so this PR no longer needs its original "player-only never restores" rule. The level `Cmd/Ctrl+B` comes back to is seeded from the restored level and kept for the session.

## Design notes

- Nested levels rather than two independent booleans: "channels hidden, categories visible" makes no sense since a category click has to bring the channels back anyway. The model follows the outside-in collapse of three-pane apps (Mail, Slack, Plex).
- The categories rail folds at level 2 **only while a category is selected**: the live root ("All Items" grid) has no channels header to host the way back, so folding there would strand the user. Level 3 folds it regardless, because the floating restore handle lives in the content area.
- `LIVE_CATEGORIES_POPOVER` (`@iptvnator/portal/shared/util`) is the DI bridge: the workspace shell provides `WorkspaceLiveCategoriesPopoverService` (CDK overlay hosting `WorkspaceContextPanelComponent` in `presentation="popover"`), the Xtream and Stalker live layouts inject it optionally and keep their plain heading without a provider.
- M3U and the unified live tab have no categories rail and treat level 2 like level 1; their code is untouched.

## Merged with #1555 (per-surface rail state)

#1555 landed while this PR was open and reworked the same service: state per surface (`m3u` / `portal` / `collection`), a workspace header toggle, the hidden-list empty state, and the legacy shared key forgotten on startup. This PR keeps that model and layers the three levels onto the `portal` surface (`areCategoriesHiddenFor`, `hideCategories` / `showCategories` / `collapse` / `expand` per surface; `toggle(surface)` returns to the level the surface collapsed from). "Show playing channel" uses `expand('portal')` so it keeps a deliberately hidden categories rail folded, and the category sort preference moved to `PortalCategorySortStateService` so the popover copy of the context panel and the retained rail agree.

## Also fixed along the way

- The channels header showed "Channels" instead of the category name: provider category ids are strings, the selection is numeric. Compared via `String()` now.
- A collapsed context panel left a 22px padding strip beside the channels rail.
- The panel toggle labels said "Hide channels list" while also hiding categories; labels and tooltips are honest now (8 new i18n keys, all 18 locales).



Docs: `docs/architecture/iptvnator-ui-guidelines.md` ("Collapsible Live Sidebar" rewritten), `docs/architecture/workspace-shell.md`. Release note: `.changes/portals-live-panel-collapse-levels.md`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

CLAUDE.md

@@ -1739,6 +1739,36 @@ preventing destination failures from penalizing the initial endpoint. Contracts:
   copy and Retry now; Stalker preserves cached account data on a failed refresh.
   Contract: `docs/architecture/host-connectivity-guard.md`.
 
+## Live TV Panel Levels
+
+Portal live layouts (Xtream `live`, Stalker `itv`/`radio`) fold their panels
+from the outside in, in three nested levels owned by `LiveSidebarState`
+(`@iptvnator/portal/shared/util`): `expanded` (categories rail + channels rail
++ player), `categories-hidden` (channels rail + player) and `collapsed`
+(player only). `LiveLayoutSidebarStateService` is the single source of truth, per
+surface (`m3u` / `portal` / `collection`; the levels apply to `portal`); the
+shell context sidebar folds the categories rail on
+`areCategoriesHiddenFor('portal')` (at level 2 only while the portal store has
+a selected category — the live root has no channels header to host the way
+back — and always at level 3), the channels rail folds on
+`isCollapsedFor('portal')`. While the rail is folded the
+channels header turns its title into a category dropdown that opens the same
+`WorkspaceContextPanelComponent` as a CDK popover through the
+`LIVE_CATEGORIES_POPOVER` token: the workspace shell provides
+`WorkspaceLiveCategoriesPopoverService` (focus-trapped `role="dialog"`,
+closed by backdrop, Escape, selection, its footer and any `NavigationStart`),
+the live layouts reach it through `createLivePanelsController()` (level
+flags, dropdown bridge and focus handoff in one shared object; the token is
+optional). `Cmd/Ctrl+B`, the header toggle and the
+floating restore handle return to the level the user collapsed from (the
+target is session-only; every level is restored as stored per surface).
+Folded rails carry `inert`, and
+`handoffFocusOnLiveSidebarChange()` / `focusIfFocusLost()` move focus to the
+replacement affordance only when the activated button was removed or inerted.
+M3U and the unified live tab have no categories rail and treat level 2 like
+level 1. Contract: `docs/architecture/iptvnator-ui-guidelines.md`
+("Collapsible Live Sidebar").
+
 ## Live Channel Return
 
 Xtream and Stalker (including radio) capture displayed playback order on explicit