feat(channels): press-and-hold priority stepper + inline-editable value

The priority arrows changed one step per click, so setting a large value meant many clicks. Now a
press-and-hold auto-repeats at an accelerating rate (a quick tap still steps once; keyboard path
kept), and the PATCH is debounced per channel so a hold sends one request with the final value, not
one per notch (flushed on unmount). The value also sits LEFT of the arrows (cursor never covers it)
and is click-to-edit — type any integer directly, negatives allowed — via an absolutely-positioned
input that overlays without disturbing the table row layout.

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

@@ -90,6 +90,7 @@
     "priorityHint": "Your ranking for this channel. Sort the feed by “Channel priority” to bring higher-priority channels to the top.",
     "raisePriority": "Raise priority",
     "lowerPriority": "Lower priority",
+    "editPriority": "Click to type a priority (negatives allowed)",
     "recent": "recent",
     "recentSyncedHint": "Recent uploads synced.",
     "recentNotSyncedHint": "Recent uploads not fetched yet.",

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

@@ -90,6 +90,7 @@
     "priorityHint": "A te rangsorod ehhez a csatornához. Rendezd a hírfolyamot „Csatorna prioritás” szerint, hogy a magasabb prioritású csatornák felülre kerüljenek.",
     "raisePriority": "Prioritás növelése",
     "lowerPriority": "Prioritás csökkentése",
+    "editPriority": "Kattints a prioritás beírásához (negatív is lehet)",
     "recent": "legutóbbi",
     "recentSyncedHint": "Legutóbbi feltöltések szinkronizálva.",
     "recentNotSyncedHint": "A legutóbbi feltöltések még nincsenek letöltve.",