Add --base-path support for reverse proxy subpath deployment

[d0nperinion]

Summary

• Add --base-path CLI flag (env: LEAFWIKI_BASE_PATH) so LeafWiki can be served from an arbitrary URL prefix (e.g. example.com/wiki/)
• Mount all backend routes under a Gin route group using the configured base path
• Scope auth and CSRF cookies to the base path
• Inject base path into index.html via tag; frontend reads it for API calls, React Router basename, asset URLs, and link resolution

Test plan

• go test ./... passes
• Without --base-path: everything works at / as before (no regression)
• With --base-path /wiki: page loads at /wiki, API at /wiki/api/config, navigation/assets/favicon/static/branding all work, cookies scoped to
  /wiki

Backend changes

• cmd/leafwiki/main.go: Added --base-path flag (string, env LEAFWIKI_BASE_PATH), normalizeBasePath() function that converts any input to /mypath
  format (or "" for root), passed to RouterOptions.BasePath
• internal/http/router.go: Added BasePath to RouterOptions. Created base := router.Group(options.BasePath) and mounted all routes (/assets, /api,
  /branding, /static, /favicon.svg) under it. NoRoute stays on router but strips basePath before checking prefixes. Replaces {{BASE_PATH}} in
  index.html. Added basePath to /api/config response. Passes cookiePath to cookie constructors.
• internal/http/middleware/auth/auth_cookie.go: Added CookiePath field, updated NewAuthCookies() to accept cookiePath parameter, replaced
  hardcoded Path: "/" with Path: a.CookiePath
• internal/http/middleware/security/csrf_cookie.go: Same pattern — CookiePath field, updated constructor, dynamic path in all cookies

Frontend changes

• ui/leafwiki-ui/index.html: Added
• src/lib/config.ts: Reads BASE_PATH from meta tag, uses it as default API_BASE_URL when VITE_API_URL is unset
• src/features/router/router.tsx: Added basename parameter, passed to createBrowserRouter()
• src/App.tsx: Imports BASE_PATH, passes to createLeafWikiRouter()
• src/features/preview/MarkdownImage.tsx: Prepends BASE_PATH to /assets/... src URLs
• src/features/preview/MarkdownLink.tsx: Prepends BASE_PATH to asset hrefs; strips BASE_PATH from window.location.pathname before resolving
  relative links
• src/lib/urlUtil.ts: stripBasePath() helper; buildEditUrl/buildViewUrl strip BASE_PATH prefix before processing

Test fixes

Updated all NewAuthCookies() and NewCSRFCookie() calls in test files to include the new cookiePath parameter ("/").

vibecoded this in a hour, looks like it works.

[perber]

Hi @d0nperinion,

Thanks for your PR.
I will take a look tomorrow.

Could you run npm run format in the leafwiki-ui directory. This should resolve the linting issues.