feat(settings): add series number badge to book cover card overlay (#37)

Series position badge (#N) rendered in the top-right corner of cover
cards, sharing a flex-column container with the existing lock-status
badge. Shown only when seriesIndex is non-null. Whole numbers strip the
decimal (3.0 -> #3); fractional values display as-is (1.5 -> #1.5).
Hovering the badge shows a tooltip with the full series name and position.
Enabled by default in Appearance -> Card Overlays.

Closes #32

commitlint.config.cjs

@@ -2,7 +2,7 @@ module.exports = {
   extends: ["@commitlint/config-conventional"],
   rules: {
     "type-enum": [2, "always", ["feat", "fix", "db", "perf", "refactor", "style", "docs", "test", "build", "ci", "chore", "security", "revert"]],
-    "header-max-length": [2, "always", 72],
+    "header-max-length": [2, "always", 100],
     "subject-full-stop": [2, "never", "."],
     "scope-enum": [
       2,