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>

docs/architecture/workspace-shell.md

@@ -146,6 +146,12 @@ default. The panel header exposes a sort menu next to category search with
 synthetic "all categories" entries stay pinned before sorted provider
 categories.
 
+On live sections the category panel can be folded away independently of the
+channels list; the folded panel is reachable as a popover from the channels
+header through the `LIVE_CATEGORIES_POPOVER` token the shell provides. The
+three nested levels, their affordances and persistence are specified in
+`iptvnator-ui-guidelines.md` ("Collapsible Live Sidebar").
+
 A category click in a LIVE section (Xtream `live`, Stalker `itv` and `radio`)
 changes only the selected category: the live layouts gate their player on the
 store's selected item, so the handler must not clear it — the channel the user