improvement(channel): bring the actions to the tabs, list every channel

Channel page: the actions sat in the header's top-right corner, a page-width
mouse trip from anything else you'd click. There are only ever two tabs, so
they ride the tab row now, right next to About.

Channel picker: drop the 50-row cap — 320 plain buttons cost nothing to render
(the feed virtualises thousands), so the whole list scrolls in the same box. The
cap was caution, not a measurement. The box gets the app's usual edge-fade to
hint at more, and a line saying "Priority first, then A-Z" — the backend orders
by `priority DESC, lower(title)`, which reads as random to anyone who hasn't set
a priority.

frontend/src/i18n/locales/en/sidebar.json

@@ -76,5 +76,5 @@
   },
   "channelSearch": "Search channels…",
   "noChannelMatch": "No channel matches.",
-  "channelMore": "Showing {{shown}} of {{total}} — search to narrow."
+  "channelOrder": "Priority first, then A–Z"
 }

frontend/src/i18n/locales/hu/sidebar.json

@@ -76,5 +76,5 @@
   },
   "channelSearch": "Csatornák keresése…",
   "noChannelMatch": "Nincs találat.",
-  "channelMore": "{{total}} közül {{shown}} látszik — keress a szűkítéshez."
+  "channelOrder": "Prioritás szerint, majd A–Z"
 }