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/swarm-ui/src/shell/LinksMenu.tsx

@@ -12,6 +12,7 @@
 // render a dead affordance" rule the old dashboard's H0M3 tiles follow
 // for Forge/Matrix.
 import { useEffect, useRef, useState } from 'preact/hooks';
+import { LinkIcon } from '@hive/shared/icons.js';
 import './LinksMenu.css';
 
 interface ServiceLink {
@@ -62,30 +63,12 @@ export function LinksMenu() {
         aria-label="swarm services"
         onClick={() => setOpen((v) => !v)}
       >
-        {/* Feather/lucide "link" glyph, not the 🔗 emoji this replaced —
-            reported live by mara: "links and settings icon styles
-            different / all different sizes". An emoji glyph is rendered
-            by the
-            OS/browser's colour-emoji font at that font's own fixed
-            metrics, ignoring `font-size`/`color` — it can never match a
-            neighbouring text-style icon (SettingsMenu's `⚙`) in size or
-            weight, on any platform. Same inline-SVG shape as
-            SettingsMenu's gear now uses, so both trigger buttons finally
-            share one rendering path (stroke, viewBox, size). */}
-        <svg
-          width="1.3em"
-          height="1.3em"
-          viewBox="0 0 24 24"
-          fill="none"
-          stroke="currentColor"
-          stroke-width="2"
-          stroke-linecap="round"
-          stroke-linejoin="round"
-          aria-hidden="true"
-        >
-          <path d="M10 13a5 5 0 0 0 7.54.54l3-3a5 5 0 0 0-7.07-7.07l-1.72 1.71" />
-          <path d="M14 11a5 5 0 0 0-7.54-.54l-3 3a5 5 0 0 0 7.07 7.07l1.71-1.71" />
-        </svg>
+        {/* `@hive/shared/icons.js`'s `LinkIcon` — reported live by mara:
+            "links and settings icon styles different / all different
+            sizes". Now shared with `SettingsMenu`'s `GearIcon` and
+            agent's own `MetaNav` trigger, one rendering path instead of
+            three separate inline copies — see that file's comment. */}
+        <LinkIcon />
       </button>
       {open ? (
         <div class="links-menu-popover" role="menu">