frontend: shared <hive-warn> component for inline warning banners

Consolidates three independently-written instances of the same thing:
.cred-warning (credentials.html, static markup), .tombstone-warn
(core.js, JS-built), .port-conflict (swarm.js, JS-built) -- the first
two differed only by an undeliberate 10% vs 8% tint, the strongest
argument this was drift rather than three genuinely different needs.

New autonomous custom element, frontend/packages/shared/src/hive-warn/,
matching the established one-component-one-dir shadow-DOM pattern
(hive-btn, hive-toast, hive-dialog). Purely presentational -- no
lifecycle beyond attaching its shadow root once, no
attributeChangedCallback needed since severity/pulse are read directly
by :host([...]) CSS selectors rather than mirrored onto an inner
element. Content passes through via a single default <slot>, so every
call site keeps writing its existing <strong>/<code> markup unchanged.

API: <hive-warn> (amber, default) / <hive-warn severity="red"> for an
active incident vs a standing caveat, plus an opt-in pulse boolean
modifier (only the port-collision banner wants it -- a banner that's
always present and always pulsing just trains you to stop seeing it).
Tint is a single canonical 8% for both severities now, replacing the
10%/8% split.

Registered once in dashboard's common.js (same side-effect-import
pattern as <hive-side-panel>) so every dashboard page picks it up
without a per-file import, since all three call sites (core.js,
swarm.js, credentials.js) already transitively import it.

Verified: npm run build clean for both dashboard and agent packages,
grepped source for leftover cred-warning/tombstone-warn/port-conflict
references (none), confirmed hive-warn/HiveWarn/hive-warn-pulse present
in every affected dist bundle, confirmed the still-live
questions-pulse keyframe (.questions) untouched.

frontend/packages/dashboard/src/core.js

@@ -50,7 +50,7 @@ function renderTombstones(s) {
   // Wording is deliberately about what the list *is* rather than what it
   // isn't: nothing records a destroy, so "container absent" is the only thing
   // the backend can actually tell.
-  const warn = el('p', { class: 'tombstone-warn' });
+  const warn = el('hive-warn', {});
   warn.append(
     el('strong', {}, 'shows every agent whose container is absent'),
     ' — not only destroyed ones. An agent part-way through being spawned ' +