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>

src/common/di.rs

@@ -955,6 +955,24 @@ impl AppServiceFactory {
         tracing::info!("Tree-ETag flush service initialized");
     }
 
+    /// Start the primary-pool saturation watchdog (Finding #3). Logs a WARN as
+    /// the user-facing pool approaches exhaustion — the early signal for raising
+    /// `max_connections` or chasing a slow query before tail latency cliffs.
+    /// Skipped when `pool_monitor_interval_secs == 0`.
+    fn start_db_pool_monitor(&self, primary_pool: &Arc<PgPool>) {
+        let interval = self.config.database.pool_monitor_interval_secs;
+        if interval == 0 {
+            return;
+        }
+        crate::infrastructure::services::db_pool_monitor::DbPoolMonitor::new(
+            primary_pool.as_ref().clone(),
+            "primary",
+            self.config.database.max_connections,
+            interval,
+        )
+        .start();
+    }
+
     /// Opens (or rebuilds) the embedded Tantivy content index. Returns the
     /// index plus a reseed flag (true when the on-disk index was missing or
     /// version-stale and must be repopulated from `storage.files`). Any
@@ -1149,6 +1167,8 @@ impl AppServiceFactory {
 
             self.start_tree_etag_flush_job(&maintenance_pool);
 
+            self.start_db_pool_monitor(&pool);
+
             self.start_content_index_job(&maintenance_pool, &core, content_index);
 
             // User-lifecycle dispatcher. Hook order is registration order;