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)

docs/04-implementation-plan.md

@@ -34,7 +34,7 @@ implementation(libs.vico.compose.m3)
 implementation(libs.vico.core)
 ```
 
-**Charts**: Vico 2.x Compose-native library. `HomeScreen` dashboard renders a pie chart from `ExpenseSummary.byCategory`. No custom Canvas drawing.
+**Charts**: Vico 2.x Compose-native library. `HomeScreen` dashboard renders a Vico `ColumnCartesianLayer` from `ExpenseSummary.byCategory`, with a small category legend (`Row { colored swatch; category name; amount }`) below the chart that carries the per-category color. Vico 2.0.0's `cartesian` package only exposes `Line` / `Column` / `Candlestick` layers (no pie), so the dashboard uses a column chart with the legend. No custom Canvas drawing.
 
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