feat(admin): search users by email + erase a bound email

Add an exact (strict) email filter to the /users list (UserFilter.EmailExact →
a kind='email' identity match) with a search input, and an 'Erase email' action on
the user card that deletes the bound email identity and its pending confirmations,
freeing the address. It refuses to remove the account's only identity
(ErrLastIdentity), which would leave it unreachable. Integration tests for both.

backend/internal/adminconsole/templates/pages/user_detail.gohtml

@@ -101,6 +101,11 @@
 {{else}}<tr><td colspan="4"><span class="note">no identities (guest)</span></td></tr>{{end}}
 </tbody>
 </table>
+{{if .HasEmail}}
+<form class="form" method="post" action="/_gm/users/{{.ID}}/remove-email" onsubmit="return confirm('Erase the email identity from this account? The address will be freed.')">
+<button type="submit">Erase email</button>
+</form>
+{{end}}
 </section>
 <section class="panel"><h2>Friends</h2>
 <table class="list">

backend/internal/adminconsole/templates/pages/users.gohtml

@@ -9,6 +9,7 @@
 {{if .Robots}}<input type="hidden" name="kind" value="robots">{{end}}
 <input name="name" value="{{.NameMask}}" placeholder="display name mask (* ?)">
 <input name="ext" value="{{.ExternalIDMask}}" placeholder="external id mask (* ?)">
+<input name="email" value="{{.EmailExact}}" placeholder="email (exact)" type="search">
 <button type="submit">Filter</button>
 </form>
 <table class="list">