fix(i18n): every label a screen reader or tooltip says is translated

- The Plex playlist rows' "reorder"/"remove", the Plex filter chip's "remove" and the player
  slider's "reset" were English literals; each now names its row or setting in HU/EN.
- The playlist show header's chevron had no accessible name; its decorative thumbnail button left
  the tab order.
- The opt-in Glass tuner panel was English only: every word is translated, and each slider's label
  now names its number box and range.
- New `local/literal-a11y-text` lint rule (with its contract test): a literal word in aria-label,
  title, placeholder, alt and the other spoken/shown attributes fails lint.

frontend/src/lib/literalA11yText.contract.test.ts

@@ -0,0 +1,33 @@
+import { fileURLToPath } from "node:url";
+
+import { ESLint } from "eslint";
+import { describe, expect, it } from "vitest";
+
+// `local/literal-a11y-text` (eslint.config.js): what a screen reader or a tooltip says is user-facing
+// text, so it goes through t() like the rest. Five English `aria-label`s and a whole English panel
+// survived every lane before it. A rule that matches nothing looks exactly like a clean codebase, so
+// this asserts it fires.
+const RULE = "local/literal-a11y-text";
+const FRONTEND = fileURLToPath(new URL("../../", import.meta.url));
+
+async function hits(source: string): Promise<number> {
+  const eslint = new ESLint({ cwd: FRONTEND });
+  const [result] = await eslint.lintText(source, { filePath: "src/components/__probe.tsx" });
+  return (result?.messages ?? []).filter((m) => m.ruleId === RULE).length;
+}
+
+describe("a literal word in an attribute the user hears or sees", () => {
+  it("is refused, in every spelling of a string", async () => {
+    expect(await hits(`export const A = () => <button aria-label="remove" />;\n`)).toBe(1);
+    expect(await hits(`export const B = () => <button title={"Collapse"} />;\n`)).toBe(1);
+    expect(await hits("export const C = () => <input placeholder={`Search`} />;\n")).toBe(1);
+    expect(await hits(`export const D = () => <img alt="A poster" src="" />;\n`)).toBe(1);
+  });
+
+  it("lets through a translation, a decorative image, a symbol and a URL shape", async () => {
+    expect(await hits(`export const E = ({ t }) => <button aria-label={t("x.y")} />;\n`)).toBe(0);
+    expect(await hits(`export const F = () => <img alt="" src="" />;\n`)).toBe(0);
+    expect(await hits(`export const G = () => <span title="×" />;\n`)).toBe(0);
+    expect(await hits(`export const H = () => <input placeholder="https://…" />;\n`)).toBe(0);
+  });
+});