Release 0.7.3

### Changed - The new chat's "What can I help with?" draws in a touch, as other titles do. For forks: `shared/ui/Text.tsx` has a new `Text`, sized on one ladder over the type scale with Tailwind's names, `xs`, `sm`, `md`, `base`, `xl`, `2xl`, and `3xl`, with a `color` and `weight`, and `shared/ui/Title.tsx` a new `Title`, whose `order` is its heading level and `size` its look. Page titles, hints, and other plain text now use them. - Each step of the type scale now carries its own line height, from 1.4 for captions to 1.2 for the largest titles, and the headings and titles their letter spacing, so text that inherited the page's 1.55 sits a little tighter in settings and admin. For forks: set them in `theme.css` as `--text-*--line-height` and `--text-*--letter-spacing`. A `leading-*` or `tracking-*` utility still overrides them. - Corners now come from one ladder, so badges and rows share one radius, as do controls and cards, each moving by under a pixel. For forks: `theme.css` has `--radius-xs` to `--radius-xl` in place of `mark`, `badge` and `row`, `small` and `card`, `panel`, and `bubble`, so `rounded-badge` is `rounded-sm`, `rounded-small` is `rounded-md`, and so on. - Padding, margins, and gaps now use whole and half spacing steps only, so about a hundred of them moved by a pixel. - Greys now come from one twelve-step scale, so a few move by a shade: the sidebar is a touch deeper in light, lines and a focused field's border a touch brighter in dark, placeholders and quiet icons a touch darker in light, and section headings and model names one grey. For forks: `theme.css` holds color in three layers, scales (`--gray-1` to `--gray-12` and hue steps such as `--green-9`), roles, and component colors made from roles, so a new accent is the accent roles pointed at another hue. `sidebar` is now `chrome`, `muted-quiet`, `section-ink`, and `tint-quiet` are `subtle`, `icon-quiet` is `faint`, `skeleton` is `surface`, and `separator` is `line`. - For forks: line heights a block sets apart from its size's own are a ladder in `theme.css`, `leading-tight`, `snug`, `normal`, and `relaxed`, replacing values such as `leading-[1.35]`, and the nested corners left as `calc()` in components are tokens: `rounded-thumb`, `bubble-quote`, `toolbar`, and `panel`. The theme test now fails on a quarter spacing step, a one-off corner, line height, or tracking, a component naming a scale step, or a role written as a raw color.