Add Unit Converter tool

[02karthik]

What is this tool?

Unit Converter — a single-file, fully offline converter for the everyday quantities people actually reach for: length, weight, temperature, volume, area, speed, time, and digital storage. Pick a category, type in either field, and the other side updates live. It's for anyone in a note or draft who needs "how many km is 5 miles?" without leaving the workspace or opening a browser tab.

Highlights:

• Two-way, live conversion — edit the From or the To value and the other recomputes instantly; a swap control flips the units.
• Equation echo + quick reference — a one-line "1 mile = 1.609344 km" readout, plus a list showing the value expressed in every unit of the category.
• Correct by construction — linear categories use base-unit factors; temperature uses explicit °C/°F/K offset formulas. Results are trimmed to sensible significant figures.
• Remembers your last category and units in localStorage under unit-converter:v1.
• On-brand — azure primary #4d63ff, cool-mist canvas, Manrope 900 display, 24px card radius, per the Hillnote design system.

Checklist

• It's a single file: unit-converter/index.html with all CSS + JS inline.
• It is fully self-contained / offline — no remote fonts, scripts, styles, images, fetch, or import. All math is done in-browser.
• No analytics, trackers, or phone-home behavior.
• Added a screenshot at screenshots/unit-converter.webp (via npm run screenshot unit-converter).
• Registered the tool in tools.json (dir = unit-converter, matches the folder).
• Bumped the top-level catalogVersion (20 → 21).
• New tool → version: 1.
• npm run validate passes locally (✓ catalog valid — 27 tool(s), catalogVersion 21).

Notes for reviewers

• Temperature is the only non-linear category; it's converted via to-Celsius / from-Celsius helpers rather than a single factor.
• Number formatting uses toPrecision(8) (stripping trailing zeros) and falls back to exponential notation for very small/large magnitudes, so results stay readable across, e.g., milligrams and tonnes.
• All output inserted into the equation echo is HTML-escaped; the quick-reference list is built with textContent.
• This branch was already sitting on the pending Metronome commit (not yet in main), so the branch-level diff includes it too. My commit Add Unit Converter tool is self-contained to the new tool — the Metronome change drops out of this diff once its own PR lands in main.

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