docs: add CHANGELOG.md (v5.0.0 entry, Keep a Changelog 1.1.0 format)

CHANGELOG.md

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+# Changelog
+
+All notable changes to markdown-converter will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [5.0.0] - 2026-06-06
+
+### Added
+- **Complete React 19 + Vite + TypeScript renderer** replacing the legacy vanilla-JS UI (Phases 1-9 of the React UI redesign)
+- Native macOS/Windows/Linux menus with command palette and keyboard shortcuts
+- Settings sheet with 5 tabs (editor, theme, keybindings, advanced, about) and 17+ persisted fields
+- Modal layer with 13 modal kinds (export PDF/DOCX/HTML/Word, batch, settings, about, welcome, confirm, ASCII gen, table gen, find in files)
+- 10 advanced tools: ASCII generator (figlet), table generator, Word export (.docx via `docx` lib), find-in-files (recursive regex), REPL (markdown snippet preview), print preview, zen mode (Esc exits), minimap, breadcrumbs-with-symbols, git status (porcelain parser)
+- Sonner toast notifications at 4 wire points (file save, open file/folder, 4 export dialogs)
+- 3 mount strategies: ModalLayer dialogs, App.tsx global overlays, editor/sidebar integrations
+- `ipc.file.writeBuffer` for renderer-side binary file output (used by Word .docx export)
+- `ipc.file.search` (recursive regex), `ipc.file.gitStatus`, `ipc.print.show`, `ipc.app.showSaveDialog`
+- 305 unit + integration tests (vitest + React Testing Library)
+- Per-package @radix-ui primitives (checkbox, dialog, select, switch, tabs, radio-group, scroll-area, slider, collapsible, label, context-menu)
+- shadcn/ui (new-york style) primitives, manually pasted (CLI broken on Node 24)
+- Feature-first main process decomposition: `src/main/{files,menu,window,word-template,utils}/` + glue files
+
+### Changed
+- **BREAKING**: Renderer is now React-only. The legacy `src/renderer.js` (5319 lines) and all vanilla-JS UI scripts/styles are removed.
+- Main process decomposed from a 4311-line `src/main.js` into feature-first modules under `src/main/`
+- New entrypoint: `src/main/index.js` (was `src/main.js`)
+- `src/index.html` (1667-line legacy orphan) removed; renderer served by `src/renderer/index.html` (Vite root)
+- IPC contract: handlers throw on error, `safeCall` catches → returns `{ ok: false, error }`. `result.ok` is at top level, NOT nested in `result.data.ok`
+- Settings store: `useSettingsStore` (zustand persist with zod validation), 17+ persisted fields
+- Modal state: `useAppStore.modal: ModalState` discriminated union with 13 kinds; `openModal` uses TS conditional types to enforce prop requirements
+
+### Removed
+- `src/renderer.js` (legacy vanilla-JS renderer, 5319 lines)
+- 8 legacy stylesheets: `src/styles.css`, `src/styles-modern.css`, `src/styles-concreteinfo.css`, `src/styles-sidebar.css`, `src/styles-zen.css`, `src/styles-welcome.css`, `src/fonts.css`
+- 5 legacy scripts: `src/command-palette.js`, `src/print-preview.js`, `src/welcome.js`, `src/zen-mode.js`, `src/wordTemplateExporter.js`
+- 2 legacy HTMLs: `src/ascii-generator.html`, `src/table-generator.html`
+- `src/main.js` (4311-line god file, replaced by `src/main/index.js`)
+- `src/index.html` (1667-line legacy orphan at project root, replaced by `src/renderer/index.html`)
+- 9 dead IPC channels from `src/preload.js`: `toggle-command-palette`, `print-preview`, `print-preview-styled`, `open-ascii-generator`, `open-table-generator`, `show-ascii-generator`, `show-ascii-generator-window`, `show-table-generator`, `show-table-generator-window`
+
+[5.0.0]: https://github.com/amitwh/markdown-converter/releases/tag/v5.0.0