fix(e4): address code-review findings (a11y sweep + dedup guards)

Resolves the 5 /code-review high findings on this branch:
- a11y sweep: extend aria-label to the remaining icon-only Plex card buttons the
  first pass missed — PlexPosterCard Info, SeasonCard watched-toggle + add-season
  (EpisodeCard already had them). "Fix a principle everywhere, in one sweep."
- new shared lib/parseId.ts `parsePositiveIntId`, used by BOTH readSelectedId
  (now guarding the stored ?pl fallback the same as the URL — was unguarded) and
  plexSubUrl's ?v=playlist decode; removes the hand-duplicated guard.
- PAGE_KEY exported from navigationInit and imported by NavigationProvider (was
  defined in both).
- hoist a `watchedLabel` const per card so title + aria-label can't drift.

+8 tests (parseId unit + guarded-stored ?pl); full FE gate green (tsc/eslint/
prettier/knip, 317 vitest).

frontend/src/lib/parseId.test.ts

@@ -0,0 +1,23 @@
+import { describe, it, expect } from "vitest";
+import { parsePositiveIntId } from "./parseId";
+
+describe("parsePositiveIntId", () => {
+  it("accepts a positive integer", () => {
+    expect(parsePositiveIntId("1")).toBe(1);
+    expect(parsePositiveIntId("42")).toBe(42);
+  });
+
+  it("rejects empty / nullish", () => {
+    expect(parsePositiveIntId("")).toBeNull();
+    expect(parsePositiveIntId(null)).toBeNull();
+    expect(parsePositiveIntId(undefined)).toBeNull();
+  });
+
+  it("rejects zero, negative, fractional, and non-numeric", () => {
+    expect(parsePositiveIntId("0")).toBeNull();
+    expect(parsePositiveIntId("-1")).toBeNull();
+    expect(parsePositiveIntId("1.5")).toBeNull();
+    expect(parsePositiveIntId("abc")).toBeNull();
+    expect(parsePositiveIntId(" ")).toBeNull(); // Number(" ")===0
+  });
+});

frontend/src/lib/parseId.ts

@@ -0,0 +1,9 @@
+/** Parse a raw value to a positive-integer id, or null if it isn't one. A bare "" gives
+ *  Number("")===0 (finite but not positive), "1.5" is fractional, "-1"/"abc" invalid — all rejected.
+ *  The single guard behind the ?pl / ?v=playlist id params AND the stored playlist selection, so a bad
+ *  value (a malformed link, a legacy/corrupted storage entry) never selects a non-existent playlist. */
+export function parsePositiveIntId(raw: string | null | undefined): number | null {
+  if (raw == null || raw === "") return null;
+  const n = Number(raw);
+  return Number.isInteger(n) && n > 0 ? n : null;
+}