feat(dashboard): support serving under URL prefix via X-Forwarded-Prefix

The Hermes dashboard previously assumed it was served at the root of its
host (e.g. https://kanban.tilos.com/). When mounted behind a path-prefix
reverse proxy (e.g. https://mission-control.tilos.com/hermes/), the SPA
404'd because:

- index.html shipped absolute /assets/index-*.js URLs
- React Router had no basename
- The plugin loader hit /dashboard-plugins/<name>/... at the root host
- CSS in the bundle had absolute url(/fonts/...) references

This patch makes the dashboard prefix-aware at runtime, no rebuild
required. The proxy injects 'X-Forwarded-Prefix: /hermes' on every
request and the Python server:

- Rewrites href/src in served index.html to '${prefix}/assets/...'
- Injects 'window.__HERMES_BASE_PATH__="${prefix}"' for the SPA to read
- Rewrites url() refs in CSS at serve time

The SPA reads window.__HERMES_BASE_PATH__ once at boot and:

- Prefixes all /api/... fetches via api.ts
- Prefixes all /dashboard-plugins/... script/css URLs in usePlugins
- Sets <BrowserRouter basename={...}> so client-side routing works

When no X-Forwarded-Prefix header is present, behavior is unchanged
(empty prefix => serves at root, kanban.tilos.com keeps working).

Refs: MC-AUTO-13

web/src/plugins/usePlugins.ts

@@ -8,7 +8,7 @@
  */
 
 import { useState, useEffect, useRef } from "react";
-import { api } from "@/lib/api";
+import { api, HERMES_BASE_PATH } from "@/lib/api";
 import type { PluginManifest, RegisteredPlugin } from "./types";
 import {
   getPluginComponent,
@@ -43,7 +43,7 @@ export function usePlugins() {
     for (const manifest of manifests) {
       // Inject CSS if specified.
       if (manifest.css) {
-        const cssUrl = `/dashboard-plugins/${manifest.name}/${manifest.css}`;
+        const cssUrl = `${HERMES_BASE_PATH}/dashboard-plugins/${manifest.name}/${manifest.css}`;
         if (!document.querySelector(`link[href="${cssUrl}"]`)) {
           const link = document.createElement("link");
           link.rel = "stylesheet";
@@ -55,7 +55,7 @@ export function usePlugins() {
       // Load JS bundle. In dev, cache-bust so Vite HMR can clear the
       // in-memory registry while the browser would otherwise never
       // re-execute a previously cached <script> URL.
-      const baseUrl = `/dashboard-plugins/${manifest.name}/${manifest.entry}`;
+      const baseUrl = `${HERMES_BASE_PATH}/dashboard-plugins/${manifest.name}/${manifest.entry}`;
       const scriptSrc = import.meta.env.DEV
         ? `${baseUrl}?hermes_dv=${Date.now()}`
         : baseUrl;