Live TV search: find shows by programme title, not just channel name

[Grimothy]

Problem

The Live TV search field only matches channel names. _filteredChannels() (flutter_client/lib/features/live_tv/live_tv_screen.dart:211-230) filters on channel.name.toLowerCase().contains(query) and nothing else, so searching for a programme ("Breaking Bad") returns nothing — you can only find a network by its name.

Proposal

Add a Shows results section to the Live TV screen, rendered above the channel content and below the category bar, so it appears identically in all three view modes (list / logo grid / EPG timeline grid).

• Reuse the existing server-side show search: XtreamService.searchEpgShows (xtream_service.dart:1303, m3u-editor's search_epg_shows action), already wrapped by AppShell._searchEpgShows (app_shell.dart:778) and currently handed only to ShowsScreen.
• Reuse the existing MediaPreviewSection rail (shared/media_browsing_widgets.dart:566) used by Home and AIOStreams.
• Each result is annotated with the show's next airing time and channel, so it's clear why it matched.
• Tapping a result opens the existing show detail screen (/dvr/shows/<normalized-title>), which already carries the DVR record actions from Allow scheduling a single (or multiple) airing from DVR Shows search, not just "Record Series" #204/Show episode names (EPG subtitle) once the server exposes them (blocked on m3ue/m3u-editor#1409) #207.

Constraints

• Channel-name filtering stays synchronous and unchanged — typing must not become network-gated on a screen that is currently instant.
• Show search is debounced with a staleness guard, mirroring the pattern already proven in ShowsScreen (shows_screen.dart:51-96).
• Show results stay out of the channel list, so List<Channel> remains the contract feeding all three view builders.