fix(browser): clearing a search or loading a saved view no longer strands the browser

[ajslater]

What

The browser could end up stranded: the Top Collection setting says Publishers, but the route is a bare nav-collection root below it (e.g. /series, no parentIds, no search). The backend sends one root crumb there, breadcrumbs.vue hides it as the current view, and its "Top" fallback only fires with parentIds. That leaves no way back up, and / resumes the same saved route. #957 fixed the backend source of this state. This PR fixes three frontend paths that reach it, all in frontend/src/stores/browser.js.

1. "Clear Filters and Search" (clearFilters(true), from empty.vue) assigned the reset search straight into state and skipped _validateSearch. Entering a search redirects to the bare root of lowestShownCollection, and nothing undid that on clear. It now asks _validateSearch for the redirect before the reset overwrites the search, then follows it instead of reloading in place.

2. loadSavedSettings called _validateAndSaveSettings and threw away the redirect it returned. Two effects:

   • A saved view without a search, loaded mid-search, stayed at /series.
   • A view with another top collection stayed on the old route. There the server's _validate_top_collection rewrote it: a Folders view loaded at the Publishers root came back as Publishers (confirmed with a throwaway backend probe that got a 303 with topCollection: publishers).

   It now goes through setSettings, like every other settings change.

3. _validateSearch only undid the search redirect when the route matched the current lowestShownCollection. Showing a deeper level mid-search (e.g. turning on Volumes while searching at /series) left the user stranded. Honoring the saved-view redirect in (2) would have opened the same hole for a saved search whose show flags differ. Clearing a search now leaves any bare imprints / series / volumes root. Bare folders / arcs / comics roots are unaffected, since searching there never redirects.

Reviewer notes

• routeWithSettings still ignores the redirect on purpose, because it pushes its own route. It is now the only caller that does.
• On the redirect branch, clearFilters and loadSavedSettings skip the trailing settings PATCH, just as setSettings already does. The browse GET after the redirect sends the full settings, and the backend saves them.
• The NEWS line is under v2.5.1 Fixes, next to fix(browser): an unresolvable collection redirects to the top, not its bare root #957's entry.

Tests

• browser-empty-clear-search.test.js: mounts empty.vue with real Vuetify and runs real store actions (createTestingPinia, stubActions: false). The router and browser API are mocked. It clicks "Clear Filters and Search" from /series, from /series/5 and at the top.
• browser-store-saved-settings.test.js: loads saved views mid-search, with a different top collection (Folders from the root, Publishers from inside a folder), with more show levels, and in place.
• browser-store-search-clear.test.js: adds a case for show flags changing mid-search, plus cases that folders/arcs/comics roots never redirect.

Each new case failed before the fix. The show-flag cases also fail with only the old _validateSearch condition put back.

make fix, make lint and make test all pass: vitest 953/953, pytest 1569 passed with 1 expected failure. Not checked in a live browser, because another dev server held the ports.

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