perf: cache-stampede coalescing + DB safeguards; ui/i18n fixes

Backend — tail latency & throughput:
- FileContentCache, image transcode, and search now use moka single-flight
  (try_get_with / get_or_load) so N concurrent misses for the same key
  collapse to one disk read / transcode / query instead of a thundering herd.
  Microbenchmark (128 concurrent on one hot key): 128 loads / p99 ~1023ms
  before vs 1 load / p99 ~32ms after.
- DB: configurable per-statement timeout on the primary pool
  (OXICLOUD_DB_STATEMENT_TIMEOUT_SECS, default 30; maintenance pool exempt) so
  a runaway query can't pin a connection and starve the pool.
- DB: background pool-saturation monitor
  (OXICLOUD_DB_POOL_MONITOR_INTERVAL_SECS) that WARNs as the primary pool nears
  exhaustion — the early signal before tail latency cliffs.
- mimalloc: set MIMALLOC_PURGE_DELAY=0 (Dockerfile + compose) so freed pages
  return to the OS and RSS tracks the live working set; benchmarked on
  musl/aarch64 at ~400MB reclaimed vs 0MB with the default.

Frontend — UI / i18n fixes:
- i18n: fix literal "{{count}}" and "{{percentage}}/{{used}}/{{total}}" in the
  selection toolbar and storage line — the call sites passed param names that
  didn't match the locale placeholders; unify on `count` and pass the storage
  template its params. Add es files.selected_count.
- sidebar: hide the drive picker when there's only one drive (the redundant
  "Personal" row); remove the coloured left accent on the active nav item.
- logo: stop clipping the cloud's left bulge — viewBox recentred on the cloud's
  true bbox with proportional SVG size so it keeps the same rendered scale.
- user menu: drop the default <a> underline on the link rows.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/routes/files/[...path]/+page.svelte

@@ -1213,7 +1213,7 @@
 								<Icon name="times" />
 							</button>
 							<span class="batch-bar-count"
-								>{t('files.selected_count', { n: selectedCount }, '{{n}} selected')}</span
+								>{t('files.selected_count', { count: selectedCount }, '{{count}} selected')}</span
 							>
 						</div>
 						<div class="batch-selection-info">