fix: tell a failed search apart from one with no results

[uuzzrm]

Closes #2336.

What changed

fetchSearXNG used to catch every failure and return [], so when SearXNG was down the client got exactly the same response as a search that simply had no results. Both cases showed the red "Search failed" alert with a retry button — someone searching for something genuinely obscure was told the search broke, and a real outage looked like an ordinary empty search.

Now:

• Server: upstream failures throw SearXNGSearchError, and /search/text / /search/images answer 503 for them. A genuinely empty result set still comes back as 200 with [].
• Client: startTextSearch / startImageSearch no longer derive failed from length === 0. An outage leaves the state failed; an empty result set leaves it completed, which renders the already-existing "No results found" alert.
• The keyword fallback in startTextSearch only runs for real empty result sets now, so an outage doesn't fire a second request.

Why a status code

The client already treats a non-OK response as a failure in searchService.performSearch, so a 503 needs no new client plumbing — the empty array keeps meaning exactly one thing.

Tests & docs

• Updated the failure-injection matrix in docs/failure-injection.md and removed the Known Limitations entry that pinned the old behavior.
• server/webSearchService.test.ts now asserts the outage rejects while a zero-result search resolves to [].
• server/searchEndpointServerHook.test.ts asserts 503 for text and image searches when SearXNG is down, and 200 with [] for a no-result search.
• client/modules/textGeneration.degradation.test.ts covers: empty result set → completed, outage → failed with no fallback, plus the image-search equivalents.
• client/modules/search.test.ts updated for searchText / searchImages propagating failures instead of collapsing them into [].

Full suite is green (vitest run), tsc passes, and Biome/knip/jscpd report no issues.

[uuzzrm]

Closing this in favor of #2377 - same approach (rethrow so an outage reaches the client as non-200 instead of an empty result list), and that PR is on the updated base with the full test/docs set and green CI. Keeps one PR for #2336.