fix(#5): address PR review — share ExpandedFab, inject GetExpenseSummary, column-chart spec

- Add ui/components/ExpandedFab.kt with ExpandedFab + MiniFab helpers; HomeScreen and ExpenseListScreen both consume it, the tab-collapsing LaunchedEffect in ExpenseListScreen is hoisted to its Content()
- Inject GetExpenseSummary in HomeScreenModel; drive summary via dateRange.flatMapLatest { getExpenseSummary.await(it) } (fixes the period-filter total-card flicker) and drop the inline combine(expenses, dateRange) recomputation
- Hoist isFabExpanded out of HomeScreenModel into HomeScreen.Content() so the FAB state is local to the composable
- Convert HomeScreenModel.exportToCsv from a callback to a suspend fun returning Result<Unit>; the screen does the snackbar dispatch on the coroutineScope
- Consolidate DateRangeOption label mapping to a single DateRangeOption.labelRes() / .labelText() pair (one source of truth)
- Rename FAB string keys to shared fab_manual / fab_import and drop the home_fab_* duplicates
- Update docs/04-implementation-plan.md and .opencode/agent/implementor.md Charts sections to reflect the Vico 2.0.0 column-chart-with-legend substitution (Vico 2.0.0 has no pie layer)

app/src/main/res/values/strings.xml

@@ -72,8 +72,6 @@
     <string name="expense_list_filter_all">All</string>
     <string name="expense_list_filter_this_week">This week</string>
     <string name="expense_list_filter_this_month">This month</string>
-    <string name="expense_list_fab_manual">Manual</string>
-    <string name="expense_list_fab_import">Import Bank Statement</string>
     <string name="expense_list_fab_add_recurring">Add Recurring</string>
     <string name="expense_list_delete_title">Delete expense?</string>
     <string name="expense_list_delete_message">This will permanently delete the expense.</string>
@@ -103,6 +101,10 @@
     <string name="add_edit_recurring_interval_monthly">Monthly</string>
     <string name="add_edit_recurring_interval_yearly">Yearly</string>
 
+    <!-- Shared FAB (issue #5) -->
+    <string name="fab_manual">Manual</string>
+    <string name="fab_import">Import Bank Statement</string>
+
     <!-- Home (issue #5) -->
     <string name="home_title">Ledgerr</string>
     <string name="home_total">Total %1$s</string>
@@ -115,8 +117,6 @@
         <item quantity="one">%d new recurring expense added</item>
         <item quantity="other">%d new recurring expenses added</item>
     </plurals>
-    <string name="home_fab_manual">Manual</string>
-    <string name="home_fab_import">Import Bank Statement</string>
     <string name="home_dashboard_empty">No expenses yet for this period</string>
     <string name="home_settings">Settings</string>
     <string name="home_export_failure">Export failed</string>