frontend: shadow-DOM upgrade for hive-dialog/hive-toast, drop global modal.css

Follow-up to the light-DOM custom-elements pilot (mara: 'that one landed
and works. i dont really like the css still being shared - id like that
to be split by component, with common stuff via @include').

<hive-dialog> and <hive-toast> now attach a shadow root and adopt a
component-scoped CSSStyleSheet built from a template-string constant in
modal.js (DIALOG_CSS / TOAST_CSS), plus a new shared/src/component-
styles.js sheet (currently just .btn) adopted alongside it via
adoptedStyleSheets -- the native equivalent of a Sass @include, no
preprocessor added. Theme vars keep resolving through the shadow
boundary since CSS custom properties inherit across it; only plain
class rules needed the explicit move.

Deleted the global shared/src/modal.css entirely and dropped its
@import from both dashboard/common.css and agent/agent.css -- nothing
outside modal.js renders the old .tc-* classes any more. The <hive-
dialog> element is now the backdrop itself (:host carries the fixed-
position/centering rules that used to be .tc-backdrop on a light-DOM
div); box/title/message/content/actions all render inside its shadow
root. <hive-toast> similarly styles :host directly instead of a light-
DOM div, with the message text placed straight into the shadow root
(no <slot> needed since there's no external light-DOM content to
project).

Public API (openDialog/themedConfirm/themedPrompt/themedToast)
unchanged -- no call-site changes needed anywhere in dashboard/agent.

Verified with a full frontend build; nix fmt clean.

frontend/packages/dashboard/src/common.css

@@ -4,7 +4,6 @@
 @import "@hive/shared/terminal.css";
 @import "@hive/shared/tabs.css";
 @import "@hive/shared/chrome.css";
-@import "@hive/shared/modal.css";
 
 /* ─── global typography ─────────────────────────────────────────────
    Element-level rules shared across all three pages (index, flow,