feat(ui): nerd font icons on hard-defined list rows, graceful degrade

Tabs, Discover categories (replacing unused placeholder glyphs), Settings
sections, and the Feed 'Fetch More' row get Nerd Font glyphs (Font Awesome PUA
codepoints). When the terminal font isn't Nerd Font capable the glyphs render
nothing at all — no tofu, no layout gaps — via supportsNerdFonts() (env
allowlist + PODTUI_NERD_FONTS=1/0 override). Documented in README (Configuration
-> Fonts).

src/utils/nerd-fonts.ts

@@ -0,0 +1,115 @@
+/**
+ * Nerd Font support detection + icon codepoints for PodTui.
+ *
+ * The app prepends Nerd Font glyphs to hard-defined list rows (tabs, Discover
+ * categories, Settings sections, the Feed "Fetch More" row). When the user's
+ * terminal font is NOT Nerd Font capable those glyphs must not render at all —
+ * no tofu boxes, no empty columns — so every call site gates the icon on
+ * `supportsNerdFonts()`.
+ *
+ * Detection is a heuristic (see `supportsNerdFonts`); the `PODTUI_NERD_FONTS`
+ * env override wins over everything so a wrong guess is always fixable.
+ * Under tmux the outer terminal decides — `TMUX` being set counts as
+ * capable (the multiplexer passes glyphs through), matching the same choice
+ * made for `screen`-prefixed TERM values. See README → Configuration → Fonts.
+ *
+ * This module is deliberately free of Solid/JSX imports so it stays
+ * unit-testable in isolation.
+ */
+
+// ── Detection ────────────────────────────────────────────────────────────────
+// Memoized: the terminal does not change mid-session, so detect once.
+let cached: boolean | null = null;
+
+/**
+ * True when the terminal is (very likely) using a Nerd Font-patched font.
+ *
+ * Order:
+ *   a. `PODTUI_NERD_FONTS` env override ("1"/"true" → true, "0"/"false" →
+ *      false) — wins over everything.
+ *   b. Allowlist: TERM_PROGRAM ∈ {iTerm.app, WezTerm, vscode, ghostty, rio,
+ *      hyper, tabby, contour}, or TERM starts with {xterm-kitty, foot,
+ *      alacritty, contour, screen} (tmux/screen passthrough — the outer
+ *      terminal decides), or WT_SESSION set (Windows Terminal), or TMUX set.
+ *      Case-insensitive.
+ *   c. Everything else (Terminal.app default SF Mono, plain xterm, unknown)
+ *      → false.
+ */
+export function supportsNerdFonts(): boolean {
+	if (cached !== null) return cached;
+
+	// a. Env override wins over everything.
+	const override = process.env.PODTUI_NERD_FONTS?.trim().toLowerCase();
+	if (override === "1" || override === "true") {
+		cached = true;
+		return cached;
+	}
+	if (override === "0" || override === "false") {
+		cached = false;
+		return cached;
+	}
+
+	// b. Allowlist.
+	const termProgram = process.env.TERM_PROGRAM?.toLowerCase() ?? "";
+	const term = process.env.TERM?.toLowerCase() ?? "";
+	const TERM_PROGRAM_ALLOWLIST: Record<string, true> = {
+		"iterm.app": true,
+		wezterm: true,
+		vscode: true,
+		ghostty: true,
+		rio: true,
+		hyper: true,
+		tabby: true,
+		contour: true,
+	};
+	const TERM_PREFIX_ALLOWLIST = [
+		"xterm-kitty",
+		"foot",
+		"alacritty",
+		"contour",
+		"screen",
+	];
+	cached =
+		TERM_PROGRAM_ALLOWLIST[termProgram] === true ||
+		TERM_PREFIX_ALLOWLIST.some((prefix) => term.startsWith(prefix)) ||
+		!!process.env.WT_SESSION ||
+		!!process.env.TMUX;
+
+	// c. Everything else falls through to false.
+	return cached;
+}
+
+// ── Icon codepoints ──────────────────────────────────────────────────────────
+// Font Awesome codepoints in the Nerd Font PUA range — stable across Nerd
+// Font versions. Keyed by the semantic names the list rows use.
+export const NF_ICONS: Record<string, string> = {
+	feed: "\uF09E",
+	shows: "\uF005",
+	discover: "\uF14E",
+	search: "\uF002",
+	player: "\uF144",
+	settings: "\uF013",
+	sync: "\uF021",
+	sources: "\uF143",
+	preferences: "\uF1DE",
+	visualizer: "\uF080",
+	downloads: "\uF019",
+	all: "\uF0CA",
+	technology: "\uF2DB",
+	science: "\uF0C3",
+	comedy: "\uF118",
+	news: "\uF1EA",
+	business: "\uF0B1",
+	health: "\uF21E",
+	education: "\uF19D",
+	sports: "\uF1E3",
+	"true-crime": "\uF00E",
+	arts: "\uF1FC",
+	more: "\uF141",
+	add: "\uF067",
+};
+
+/** Glyph for a named icon when Nerd Fonts are supported, else "". */
+export function nfIcon(name: string): string {
+	return supportsNerdFonts() ? NF_ICONS[name] ?? "" : "";
+}