docs: add complete page-by-page handbook (HANDBOOK.md)

A detailed walkthrough of every page + settings section: what each card,
chart, metric, toggle, and setting shows, with the exact formula and data
source for each. Covers all 23 surfaces (Dashboard, Spending/Income, Trends,
Net Worth & investments, Tax/GST, FIRE/Goals, recurring/alerts, Settings).

Generated from the live frontend source post-v2.17 migration, so each
section's 'Reads from' line names the backend endpoint/hook feeding it --
doubles as a data-consistency reference (which metric is computed once on the
server vs still client-side). Cross-linked from README, CLAUDE.md, and PAGES.md
(which stays as the shorter data-catalog).

Author: Sagargupta16

CLAUDE.md

@@ -94,7 +94,7 @@ Layered architecture:
 
 ### Frontend (`frontend/src/`)
 
-- **`pages/`** - 25 page components (24 feature pages + `MorePage` for phone grid nav), all lazy-loaded via `React.lazy` for code splitting. Pages import directly (no barrel re-export). **Structure convention**: multi-file pages use kebab-case directories (`bill-calendar/`, `year-in-review/`, `tax-planning/`, `trends-forecasts/`, `comparison/`, `goals/`, `income-expense-flow/`, `settings/`, `subscription-tracker/`) -- each containing `PageName.tsx` + `use<Page>.ts` + `types.ts` + `*utils.ts` + `components/` subfolder. Single-file pages use PascalCase (`DashboardPage.tsx`, `BudgetPage.tsx`, etc.). Settings uses `sections/` (instead of `components/`) because "section" is the domain term. `settings/sectionPrimitives.tsx` provides shared Section/FieldLabel/FieldHint primitives. See [`docs/PAGES.md`](docs/PAGES.md) for a data-focused catalog of what each page shows.
+- **`pages/`** - 25 page components (24 feature pages + `MorePage` for phone grid nav), all lazy-loaded via `React.lazy` for code splitting. Pages import directly (no barrel re-export). **Structure convention**: multi-file pages use kebab-case directories (`bill-calendar/`, `year-in-review/`, `tax-planning/`, `trends-forecasts/`, `comparison/`, `goals/`, `income-expense-flow/`, `settings/`, `subscription-tracker/`) -- each containing `PageName.tsx` + `use<Page>.ts` + `types.ts` + `*utils.ts` + `components/` subfolder. Single-file pages use PascalCase (`DashboardPage.tsx`, `BudgetPage.tsx`, etc.). Settings uses `sections/` (instead of `components/`) because "section" is the domain term. `settings/sectionPrimitives.tsx` provides shared Section/FieldLabel/FieldHint primitives. See [`docs/HANDBOOK.md`](docs/HANDBOOK.md) for the full card-by-card walkthrough of every page + setting (formula + data source per metric), or [`docs/PAGES.md`](docs/PAGES.md) for the shorter data-focused catalog.
 - **`components/`** - Organized by domain: `analytics/` (chart components including `CategoryBreakdown` for shared category treemaps), `chat/` (ChatWidget, ChatPanel, ChatMessage, useChat hook for AI chatbot), `layout/` (AppLayout, Sidebar with ProfileModal), `shared/` (reusable components like MetricCard, ProgressBar, Sparkline, AnalyticsTimeFilter, ProtectedRoute, ProfileModal, ChunkErrorBoundary, EmptyState, LoadingSkeleton, QuickInsights), `transactions/`, `upload/`, `ui/` (base primitives including ChartContainer, PageContainer, PageHeader, Spinner, DataTable, ConfirmDialog, CollapsibleSection, plus chartDefaults helpers `referenceLine`/`currencyTooltipFormatter`).
 - **`hooks/`** - Custom React hooks. `useAnalyticsTimeFilter` encapsulates time-filter state (view mode, date range, FY) shared across all analytics pages. `useChartDimensions` provides responsive chart sizing. `hooks/api/` contains TanStack Query hooks for API calls, configured with `staleTime: Infinity` and `gcTime: 1 hour`.
 - **`services/api/`** - Axios-based API client. Axios interceptor auto-attaches JWT `Authorization` header. `aiConfig.ts` handles AI provider configuration CRUD.

README.md

@@ -203,7 +203,7 @@ npx -y @mermaid-js/mermaid-cli -i docs/images/<name>.mmd -o docs/images/<name>.s
 
 ## Pages
 
-Every page focuses on a specific question you'd ask about your money. For the **detailed data catalog** — what each page shows, where the numbers come from, and what decisions it helps you make — see **[docs/PAGES.md](docs/PAGES.md)**.
+Every page focuses on a specific question you'd ask about your money. The **[Complete Handbook](docs/HANDBOOK.md)** walks through every page card-by-card — what each metric, chart, and setting shows, with the exact formula and data source for each. For the shorter **data catalog** version, see **[docs/PAGES.md](docs/PAGES.md)**.
 
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