feat: Wire uMod browse proxy and custom plugin upload

Backend:
- GET /plugins/browse proxies uMod search.json filtered to Rust category,
  with 5-minute in-memory Map cache to avoid hammering the upstream API
- POST /plugins/upload accepts .cs files up to 5 MB via multipart, persists
  to plugin_registry, and dispatches plugin_upload action over NATS so the
  companion agent can write the file to the game server
- Legacy GET /plugins/search stub preserved (now directs callers to /browse)
- FileInterceptor + @UploadedFile follow the existing maps upload pattern

Frontend:
- useApi composable gains upload() method for multipart/form-data requests
  (omits Content-Type so the browser sets the correct multipart boundary)
- plugins store adds browseUmod() calling GET /plugins/browse and
  uploadPlugin() calling POST /plugins/upload with FormData;
  UmodPlugin and UmodBrowseResult TypeScript interfaces exported
- PluginsView Browse tab now calls browseUmod() through the backend proxy
  (no cross-origin requests to uMod directly); results show title,
  downloads_shortened, and latest_release_version_formatted from the
  real uMod payload
- New Upload Custom tab: drag-and-drop or click file input for .cs files,
  client-side extension/size validation, spinner during upload, success
  toast + auto-switch to Installed tab on completion

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/stores/plugins.ts

@@ -3,10 +3,39 @@ import { ref } from 'vue'
 import { useApi } from '@/composables/useApi'
 import type { PluginEntry } from '@/types'
 
+export interface UmodPlugin {
+  name: string
+  title: string
+  slug: string
+  author: string
+  description: string
+  downloads: number
+  downloads_shortened: string
+  download_url: string
+  latest_release_version: string
+  latest_release_version_formatted: string
+  icon_url: string
+  url: string
+  tags_all: string
+  watchers_shortened: string
+  created_at: string
+  updated_at: string
+}
+
+export interface UmodBrowseResult {
+  current_page: number
+  data: UmodPlugin[]
+  last_page: number
+  per_page: number
+  total: number
+}
+
 export const usePluginStore = defineStore('plugins', () => {
   const plugins = ref<PluginEntry[]>([])
   const searchResults = ref<any[]>([])
+  const browseResults = ref<UmodBrowseResult | null>(null)
   const isLoading = ref(false)
+  const isBrowseLoading = ref(false)
   const api = useApi()
 
   async function fetchPlugins() {
@@ -18,7 +47,7 @@ export const usePluginStore = defineStore('plugins', () => {
     }
   }
 
-  async function installPlugin(data: { plugin_name: string; source: string }) {
+  async function installPlugin(data: { plugin_name: string; umod_slug?: string; source: string }) {
     await api.post('/plugins/install', data)
     await fetchPlugins()
   }
@@ -37,6 +66,7 @@ export const usePluginStore = defineStore('plugins', () => {
     await fetchPlugins()
   }
 
+  // Legacy — kept for backwards compatibility, routes to browse
   async function searchPlugins(query: string) {
     isLoading.value = true
     try {
@@ -46,15 +76,38 @@ export const usePluginStore = defineStore('plugins', () => {
     }
   }
 
+  async function browseUmod(query: string, page = 1, sort = 'downloads') {
+    isBrowseLoading.value = true
+    try {
+      const params = new URLSearchParams({ page: String(page), sort })
+      if (query.trim()) params.set('query', query.trim())
+      browseResults.value = await api.get<UmodBrowseResult>(`/plugins/browse?${params.toString()}`)
+    } finally {
+      isBrowseLoading.value = false
+    }
+  }
+
+  async function uploadPlugin(file: File): Promise<PluginEntry> {
+    const formData = new FormData()
+    formData.append('file', file)
+    const result = await api.upload<PluginEntry>('/plugins/upload', formData)
+    await fetchPlugins()
+    return result
+  }
+
   return {
     plugins,
     searchResults,
+    browseResults,
     isLoading,
+    isBrowseLoading,
     fetchPlugins,
     installPlugin,
     uninstallPlugin,
     reloadPlugin,
     updatePluginConfig,
     searchPlugins,
+    browseUmod,
+    uploadPlugin,
   }
 })