Restore /tools page with browser-only JS toolboxes

[DR-GRIEZEL]

Motivation

• Restore the previously-removed /tools/ page and provide a zero-backend, browser-only suite of developer utilities implemented in JS.
• Replace the old/static page with an interactive, modular page that keeps logic client-side and avoids server/API dependencies.

Description

• Added the Tools nav page at src/html/nav/tools.md which loads a single module script and provides five panels (Regex, JSON, Timestamp, Finance, TradingView embeds).
• Implemented src/assets/js/tools.js containing pure helper functions and DOM glue for: regex playground (presets, matches, replacement preview), JSON tools (pretty/minify/validate, JSON↔YAML, JSONPath lookup), timestamp toolbox (Unix/ISO/local, relative, duration math, timezone compare), finance mini-tools (position sizing, percent/log return, compounding, drawdown recovery, Sharpe-ish estimator), and TradingView embed helpers.
• Added src/assets/css/tools.css for panel layout and controls styling.
• Added unit tests src/test/tools.test.js covering the exported pure helpers from tools.js to keep logic testable and modular.

Testing

• Ran formatting: npx prettier --write src/html/nav/tools.md src/assets/js/tools.js src/assets/css/tools.css src/test/tools.test.js (completed).
• Ran lint: npm run lint (no errors).
• Ran unit tests: npm test (Vitest); all test files passed: 16 test files, 147 tests total, and the new src/test/tools.test.js passed.

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Codex Task