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/lib/styles/ported/sidebar.css

@@ -91,8 +91,10 @@
 }
 
 .logo svg {
-    width: 22px;
-    height: 22px;
+    /* 25px (not 22) so the cloud keeps its rendered scale after the viewBox
+       grew 280→320 to stop clipping its left bulge: 25/320 ≈ 22/280. */
+    width: 25px;
+    height: 25px;
     fill: var(--color-sidebar-text-active);
 }
 
@@ -147,13 +149,10 @@
 .nav-item.active {
     background-color: var(--color-sidebar-active-bg);
     color: var(--color-sidebar-text-active);
-    border-left-color: var(--color-accent);
     font-weight: var(--weight-semibold);
-
-    [dir="rtl"] & {
-        border-left-color: transparent;
-        border-right-color: var(--color-accent);
-    }
+    /* No coloured left accent — the tinted background + bold weight already
+       read as "active". The base `border-left: 3px solid transparent` is kept
+       on every row so removing the accent causes no horizontal text shift. */
 }
 
 .nav-item i,

frontend/src/lib/styles/ported/userMenu.css

@@ -177,6 +177,10 @@
     cursor: pointer;
     transition: background 0.15s ease;
     text-align: left;
+    /* The link rows (`<a class="user-menu-item">` — admin panel, groups,
+       profile) are styled as menu rows, not hyperlinks; drop the default
+       <a> underline so they match the <button> rows. */
+    text-decoration: none;
 }
 
 .user-menu-item:hover,