feat(ui): forge user lookup — assignee combobox, @-mentions, repo-scoped user search

Repo-scoped assignable-user search for GitHub, GitLab, and Gitea, surfaced as
an assignee combobox in the metadata editor and @-mention autocomplete in
forge comment/reply/review surfaces.

- server: GET /api/{provider}/users/search (assignees / project members),
  query + directory/override repo resolution, 429 -> 503, connected:false
  degradation; GitLab assignee writes resolve login -> ID server-side
- wire: searchUsers (+ searchLabels/milestones/branches/tags) on the three
  API clients with tests
- facade: userSearch capability (all three), searchUsers adapters,
  mapGithubAssignee/mapGitlabMember/mapGiteaAssignee -> ForgeUser
- ui: ForgeLookupCombobox (keyboard nav, debounced 30s-TTL cache,
  connected-only caching), ForgeMentionTextarea (@ token parsing, caret
  restore), free-text fallback when lookup is unavailable; i18n in 12 locales
- extras sharing the same infrastructure: GitLab create-issue dialog and
  label/milestone/branch/tag lookups in the metadata editor

packages/web/server/lib/gitea/client.js

@@ -265,6 +265,8 @@ export function createGiteaClient({ token, baseUrl }) {
       request(`/repos/${owner}/${repo}/issues/${number}/comments`, { query: params }),
     createIssueComment: (owner, repo, number, body) =>
       request(`/repos/${owner}/${repo}/issues/${number}/comments`, { method: 'POST', body: { body } }),
+    createIssue: (owner, repo, params) =>
+      request(`/repos/${owner}/${repo}/issues`, { method: 'POST', body: params }),
     updateIssue: (owner, repo, number, params) =>
       request(`/repos/${owner}/${repo}/issues/${number}`, { method: 'PATCH', body: params }),
     milestones: (owner, repo, params = {}) =>
@@ -295,6 +297,12 @@ export function createGiteaClient({ token, baseUrl }) {
       request(`/repos/${owner}/${repo}/pulls/${number}/merge`, { method: 'POST', body }),
     branches: (owner, repo, params = {}) =>
       request(`/repos/${owner}/${repo}/branches`, { query: params }),
+    // Assignable users (collaborators with role access + org members) are the
+    // mention/assign candidate set; Gitea mirrors the GitHub assignees route.
+    assignees: (owner, repo, params = {}) =>
+      request(`/repos/${owner}/${repo}/assignees`, { query: params }),
+    tags: (owner, repo, params = {}) =>
+      request(`/repos/${owner}/${repo}/tags`, { query: params }),
   };
 }