Persist audiobooks list/grid view mode per grouping

[kevinheneveld]

Summary

• The audiobooks library toolbar exposes a list/grid view toggle, persisted to localStorage. Previously the load-from-storage was nested inside an `if (scrollContainer.value)` block that only runs for the Books grouping — so switching to list mode while grouped by Author or Series, then navigating away, would silently revert to grid.
• This switches to three per-grouping keys (`listenarr.viewMode.books`, `.authors`, `.series`) and reads the relevant one unconditionally on mount + whenever `groupBy` changes. So each grouping independently remembers list vs grid — useful for users who want, e.g., a list of authors but cards for books.
• A one-time migration seeds all three keys from the legacy `listenarr.viewMode` value the first time it's read, so existing users keep their preference.

Implementation notes

• Persistence watch moved out from inside `onMounted`'s scrollContainer guard to a top-level watch, so it fires regardless of grouping.
• Removed the now-unused `stopPersistViewModeWatch` cleanup handle.
• Initial viewMode is set inline at declaration: `ref<...>(loadViewModeFor(groupBy.value))`.
• Migration is additive — the legacy key is never deleted, just no longer read once the per-grouping keys exist.

Test plan

• `cd fe && npm run test:unit` — 350 / 350 passing (2 new tests: per-grouping persistence + legacy migration)
• `npx vue-tsc --noEmit -p tsconfig.app.json` — clean
• `cd tests && dotnet test` — passing (no backend changes)

Closes kevinheneveld#3

[therobbiedavis]

@kevinheneveld Hey there! I appreciate the PR's however I do want to let you know that we're focusing on some foundational changes first and you'll need to rebase your PRs after those are merged, so you may not want to open too many at this time.

[kevinheneveld]

Got it, sorry — opened #591 (a bug fix for automatic search matching music albums) before I saw this note. Happy to wait. I'll convert these to draft so they're out of your review queue until the foundational changes land, and I'll rebase them then. No rush from my end.

[kevinheneveld]

Quick update — all five of my open PRs (#580, #585, #589, #590, #591) have been rebased onto canary (v0.4.1). #585, #589, #590, #591 are still drafted per your earlier note; #580 is the nzbget auth fix and is ready when you have time. Whenever the foundation phase feels open enough for the feature PRs again, happy to undraft any/all of them. No urgency.