Phase C: Add Manual Journal Entries (double-entry GL)

- JournalEntry + JournalEntryLine entities with Draft/Posted/Reversed lifecycle
- JournalEntryStatus enum (Draft, Posted, Reversed)
- Migration AddJournalEntries: two new tables with self-referencing reversal FK
- IUnitOfWork/UnitOfWork wired with JournalEntries + JournalEntryLines repos
- ApplicationDbContext: DbSets, tenant query filters, reversal FK config
- LedgerService: JE lines added as 10th source in GetAccountLedgerAsync and ComputePriorBalanceAsync
- JournalEntriesController: Index (All/Draft/Posted tabs), Create, Details, Post, Reverse, Delete
- Views: Index, Create (dynamic balanced line grid with running debit/credit totals), Details
- journal-entry-create.js: dynamic line management with balance indicator
- Nav: Journal Entries added to Finance section in _Layout

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Web/Views/Shared/_Layout.cshtml

@@ -1130,6 +1130,10 @@
                             <i class="bi bi-journal-bookmark"></i>
                             <span>Chart of Accounts</span>
                         </a>
+                        <a asp-controller="JournalEntries" asp-action="Index" class="nav-link">
+                            <i class="bi bi-journal-text"></i>
+                            <span>Journal Entries</span>
+                        </a>
                         if (hasReports)
                         {
                             <a asp-controller="AccountingExport" asp-action="Index" class="nav-link">