feat: labeled loading spinners + [Fetch More] pagination on the feed list

Loading indicators: the braille spinner now carries a contextual label
(Refreshing…, Fetching…, Loading more…, Discovering…, Searching…) and is
shown in every loading state that previously rendered nothing — Discover
results, Search results fallback, and the empty Feed list.

Feed pagination: a focusable "[Fetch More]" row at the bottom of the flat
feed list advances every feed's loaded window by 50 episodes via the new
loadMoreAllFeeds/hasMoreAcrossAll store API. Behavior is a setting
(Fetch More: manual|auto, default manual) persisted in config.json; auto
fetches when focus reaches the bottom row. The button row is excluded
from episode focus so no episode is double-highlighted while it is active.

src/pages/Settings/PreferencesPanel.tsx

@@ -115,5 +115,19 @@ export function usePreferencesItems(): SettingItem[] {
 					autoJumpToPlayer: !prefs().autoJumpToPlayer,
 				}),
 		},
+		{
+			id: "fetchMore",
+			label: "Fetch More",
+			kind: "select",
+			display: () => (prefs().fetchMoreMode === "auto" ? "Auto" : "Manual"),
+			help: () =>
+				`How the Feed list loads older episodes.\nManual: a "[Fetch More]" button at the bottom of the list.\nAuto: fetches automatically when reaching the bottom.\nType: select\nDefault: manual\nCurrent: ${prefs().fetchMoreMode === "auto" ? "Auto" : "Manual"}\nCycle with j/k; Enter to apply.`,
+			cycle: (dir) => {
+				const modes: Array<"manual" | "auto"> = ["manual", "auto"];
+				const idx = modes.indexOf(prefs().fetchMoreMode ?? "manual");
+				const next = modes[(idx + dir + modes.length) % modes.length];
+				app.updatePreferences({ fetchMoreMode: next });
+			},
+		},
 	];
 }