icons: restore inline-SVG link/gear icons, share them across all three pages

Fixes a regression + a longer-standing inconsistency (mara: "agent
page link icons is different from swarm ui, settings icon looks weird
since component extract").

- The `SettingsMenu` shared-component extraction swapped swarm-ui's
  original inline-SVG gear trigger for a plain `⚙` text glyph (matching
  agent's `MetaNav`, which was itself still on the `🔗` emoji at the
  time). An emoji/text glyph is rendered by the OS/browser's own font
  at that font's fixed metrics — it can never match a neighbouring
  icon in size or weight, and some codepoints (the gear included)
  aren't even reliably covered by every font.
- New `@hive/shared/icons.js` (`GearIcon`, `LinkIcon`) — the exact SVG
  markup that used to live only in swarm-ui's `SettingsMenu`/
  `LinksMenu` as two separate inline copies, now the one shared source.
- `SettingsMenu` (shared) uses `GearIcon`; agent's `MetaNav` and
  swarm-ui's `LinksMenu` both use `LinkIcon` — three consumers, one
  rendering path, matching stroke/viewBox/size everywhere.

Verified with real screenshots on both agent and swarm-ui — both
trigger icons render as the same crisp line-icon style now.

frontend/packages/agent/src/components/MetaNav.tsx

@@ -1,5 +1,10 @@
 // <MetaNav> — the header's meta-nav trigger: a single fixed-size Badge
-// ("🔗") in the pills cluster that opens a popover listing
+// (`@hive/shared/icons.js`'s `LinkIcon` — an emoji glyph here can't
+// match a neighbouring icon's size/weight on any platform, see that
+// file's comment; mara: "agent page link icons is different from
+// swarm ui" was this trigger still on the old `🔗` emoji after
+// swarm-ui's own link trigger had already moved to the SVG) in the
+// pills cluster that opens a popover listing
 // stats/screen/forge/config + any `hyperhive.dashboardLinks` extras,
 // sourced from the backend's `agent_links()` (the single source of
 // truth — same list also feeds the dashboard card's icon strip) — plus
@@ -25,6 +30,7 @@
 // `LoginFlow`'s `.login-card` already made for the same reason.
 import { useEffect, useRef, useState } from 'preact/hooks';
 import { Badge } from '@hive/shared/badge.js';
+import { LinkIcon } from '@hive/shared/icons.js';
 import type { AgentLink } from '../types.js';
 import './MetaNav.css';
 
@@ -67,7 +73,7 @@ export function MetaNav({ links, forgePublicUrl, dashboardBase }: MetaNavProps)
 
   return (
     <div class="meta-nav-anchor" ref={rootRef}>
-      <Badge value="🔗" title="agent links" onClick={() => setOpen((o) => !o)} expanded={open} />
+      <Badge value={<LinkIcon />} title="agent links" onClick={() => setOpen((o) => !o)} expanded={open} />
       {open ? (
         <div class="meta-nav-popover" role="menu" aria-label="agent links">
           <a