feat(#617): skeleton loaders instead of bare spinner on list screens (#625)

Adds SkeletonListState to StateViews.kt — an animated shimmer
placeholder list whose geometry matches a typical list row (avatar circle
+ title line + subtitle line). The shimmer colors derive from
MaterialTheme.colorScheme.surfaceVariant (theme-aware, works on
light/dark/AMOLED/Material You). Sweeps horizontally via an
infiniteTransition tween — no new dependencies, pure Compose foundation
animations.

Wired SkeletonListState into 22 list-loading call sites across 22 screens:
skills, cron, plugins, mcp, toolsets, webhooks, system, sessions (x2),
config (x2 — list + YAMLEditor), model, profiles, keys, channels,
achievements, gateways, kanban, logs, analytics, providers,
processes, hub skills tab, installed skills tab (list branch).

Preserved LoadingState() (centered spinner) where the issue explicitly
calls for it:
- PairingScreen (one-shot auth flow)
- SkillEditorDialog / ModelPickerDialog / CronJobEditorDialog interiors
- SkillPreviewDialog and SkillEditorDialog inline-loading branches in
  SkillsScreen.kt (lines 790, 914)
- InstalledSkillsTab dialog-interior branch (line 393)

SkeletonListState testTag: "skeleton_list_state".

Verified: ktlintCheck, assembleDebug, testDebugUnitTest all green locally.

Co-authored-by: Hy4ri <hy4ri@users.noreply.github.com>

app/src/main/java/com/m57/hermescontrol/ui/system/SystemScreen.kt

@@ -78,9 +78,9 @@ import com.m57.hermescontrol.ui.common.EmptyState
 import com.m57.hermescontrol.ui.common.ErrorState
 import com.m57.hermescontrol.ui.common.HermesScaffold
 import com.m57.hermescontrol.ui.common.InfoRow
-import com.m57.hermescontrol.ui.common.LoadingState
 import com.m57.hermescontrol.ui.common.NavIcon
 import com.m57.hermescontrol.ui.common.SectionHeader
+import com.m57.hermescontrol.ui.common.SkeletonListState
 import com.m57.hermescontrol.ui.common.StatCard
 import com.m57.hermescontrol.ui.common.StatusBadge
 import com.m57.hermescontrol.ui.common.StatusBadgeType
@@ -252,7 +252,7 @@ fun SystemScreen(
         when {
             state.isLoading && state.stats == null && state.doctorReport == null &&
                 state.status == null -> {
-                LoadingState()
+                SkeletonListState()
             }
 
             state.errorMessage != null -> {