frontend: split hive-agent-menu's generic dropdown mechanics into a shared hive-menu component

<hive-agent-menu> bundled two concerns: the agent-specific trigger/item
list, and generic "click a trigger, get a positioned dropdown" mechanics
(shadow attach, open/close, singleton close-on-open coordination,
outside-click/Escape handling). Pulled the latter out into a new
@hive/shared/hive-menu.js (<hive-menu>), following the established
per-component-directory + ._opts-before-append shadow-DOM pattern
(<hive-dialog>). <hive-agent-menu> now just builds the "⋮" trigger and
the action list and hands them to an internal <hive-menu> instance.

<hive-menu> takes ownership of every <hive-menu> instance in the app for
singleton coordination (closeAllMenus, renamed from closeAllAgentMenus)
— a deliberate widening from the old per-agent-menu-only tracking, since
the mechanism was never agent-specific to begin with.

The one subtlety worth spelling out: <hive-menu> projects the caller's
opaque trigger/content nodes via named <slot>s rather than moving them
into its own shadow root. That's load-bearing, not cosmetic — if it
re-parented them into its own shadow tree instead, <hive-agent-menu>'s
own classes (.agent-menu-btn, .agent-menu-item, ...) would stop applying,
since a <style> only styles elements within the same shadow tree/document
it's part of, and only slotting (not re-parenting) keeps the caller's
nodes in the caller's own tree for styling purposes. That in turn made
<hive-agent-menu>'s own shadow root redundant once it wasn't the thing
positioning or owning open/close state anymore, so it's dropped in favor
of a plain light-DOM element styled by dashboard.css (already the one
page it renders on) — hive-agent-menu.css is gone, its rules folded into
dashboard.css's per-agent-menu section, minus the positioning rules that
moved into hive-menu.css as the new generic `.menu-dropdown` wrapper.

Verified with a standalone esbuild bundle + a cached nix chromium driven
over raw CDP (no puppeteer/playwright/python3 available): hover-reveal
opacity, dropdown open/close/positioning, outside-click/Escape dismissal,
and cross-instance singleton coordination all behave identically to
before the split.

frontend/packages/shared/src/hive-menu/hive-menu.css

@@ -0,0 +1,32 @@
+/* hive-menu.css — scoped stylesheet for the generic <hive-menu>
+   shadow-DOM custom element (hive-menu.js). Loaded as raw text at build
+   time (esbuild's `text` loader) and appended as a <style> element
+   inside the shadow root — see @hive/shared/shadow-css.js's header
+   comment for why a plain <style> tag and not adoptedStyleSheets.
+
+   `:host` carries only the one truly generic requirement: it's the
+   containing block for the shadow tree's absolutely-positioned dropdown
+   — a shadow host is the containing block for its own shadow tree's
+   positioned descendants, exactly like any other positioned ancestor in
+   the flat tree. Any *layout* role the host plays in a particular
+   caller's own flex row (e.g. <hive-agent-menu>'s placement in
+   `.container-row`) is caller-specific and lives in the caller's own
+   stylesheet instead, not here.
+
+   `.menu-dropdown` is the generic positioning box wrapping the caller's
+   opaque `content` node (projected in via `<slot name="content">` — see
+   hive-menu.js). Only position/z-index/visibility live here; the
+   button/item-row *visual* styling (colors, fonts, hover states) is
+   presentational content the caller's own trigger/content nodes carry,
+   so it lives in the caller's own stylesheet — `<hive-menu>` never sees
+   those class names, only the opaque nodes handed to it. */
+
+:host {
+  position: relative;
+}
+.menu-dropdown {
+  position: absolute;
+  right: 0;
+  top: calc(100% + 2px);
+  z-index: 50;
+}