frontend: unify pill/chip/badge shape into shared CSS

Per mara's steer on #3053 ("chip/pill/badge is the same if you squint
... theme unification is part of the goal ... make it common css
instead of component, thats fine. but make them look unified"):
shared CSS, not a JS component.

New @hive/shared/pill.css defines the shape (border/border-radius/
padding/font-size/letter-spacing — color stays per-site, that's the
meaningful semantic part) in two tiers matching what was already
organically in use: `.hive-pill` (primary state chips) and
`.hive-pill-sm` (secondary meta chips). Every pre-existing classname
that drew its own copy of this shape (dashboard's `.badge` family,
agent's `.status-badge`/`.state-badge`/`.header-pill`/`.ctx-badge`/
`.model-chip`/`.effort-chip`) is folded straight into the same
selector groups, so no markup or JS changes were needed anywhere —
every render call site keeps constructing the exact same classnames
it always did.

Most visible consequence: dashboard's `.badge` family moves from its
own shape (2px square corners, uppercase, tighter padding) onto the
shared rounded-pill shape + agent's "sm" tier sizing, matching the
"look unified" ask directly. `npm run build` clean across all three
packages.

frontend/packages/shared/package.json

@@ -16,6 +16,7 @@
     "./base.css": "./src/base.css",
     "./terminal.css": "./src/terminal/terminal.css",
     "./chrome.css": "./src/chrome.css",
+    "./pill.css": "./src/pill.css",
     "./forms.js": "./src/forms.js",
     "./prefs.js": "./src/prefs.js",
     "./dom.js": "./src/dom.js",