fix: correct bulk session delete endpoint + surface deleted count (#653)

* fix: correct bulk session delete endpoint + surface deleted count

- Fix Retrofit @POST path from 'api/sessions/delete' to 'api/sessions/bulk-delete'
- Change response type from Response<Unit> to Response<BulkDeleteResponse>
- Add BulkDeleteResponse model (ok, deleted) matching backend contract
- Surface backend 'deleted' count in toast message instead of using client-side count

Backend contract (web_server.py:10750):
  POST /api/sessions/bulk-delete
  Body: { "ids": [...], "delete_all": false }
  Response: { "ok": true, "deleted": <count> }

Fixes #651

* fix: replace client-side session list filter with full refresh after bulk delete

- Call loadSessions() + loadStats() after successful bulk delete instead of
  filtering sessions/searchResults locally via 'ids' — fixes state desync
  when backend deletes fewer sessions than requested (partial success) and
  is correct for delete_all=true where ids may be empty
- Show 'No sessions were deleted' toast when deletedCount == 0
- Extract toastMsg to separate val to satisfy ktlint lint rules

Addresses review findings: UI state desync, delete_all handling, hardcoded
pluralizing, and zero-deletion UX.

---------

Co-authored-by: M57 <Hy4ri@users.noreply.github.com>

app/src/main/java/com/m57/hermescontrol/data/model/SessionListResponse.kt

@@ -41,6 +41,12 @@ data class BulkDeleteRequest(
     val delete_all: Boolean = false,
 )
 
+@Serializable
+data class BulkDeleteResponse(
+    val ok: Boolean = false,
+    val deleted: Int = 0,
+)
+
 @Serializable
 data class PruneRequest(
     val days: Int,