fix(admin): align admin.dashboard factory with HTTP plugin patterns, remove duplicated merge logic

[Copilot]

The admin.dashboard module factory was incomplete compared to the HTTP plugin's staticFileServerFactory: missing spaFallback, cacheMaxAge, router support, and config.ResolvePathInConfig path resolution. Additionally, mergeAdminConfig/injectUIRoot were duplicated between cmd/server/main.go and the admin plugin's wiring hook, resulting in double execution with a defensive skip.

Changes

plugins/admin/plugin.go

• admin.dashboard factory: add config.ResolvePathInConfig for relative path resolution; spaFallback (defaults true), cacheMaxAge, and router config params — matching staticFileServerFactory in the HTTP plugin
• injectUIRoot: extend to also override root on admin.dashboard modules (previously only static.fileserver)

plugins/admin/plugin_test.go

• Add TestWiringHookInjectsUIDirForAdminDashboard: verifies UIDir override applies to admin.dashboard modules

cmd/server/main.go

• Remove duplicated mergeAdminConfig + injectUIRoot functions and their call in setup() — the admin plugin's wiring hook is now the single merge point

// admin.dashboard factory now mirrors staticFileServerFactory:
spaFallback := true  // on by default for SPA admin UI
if sf, ok := cfg["spaFallback"].(bool); ok {
    spaFallback = sf
}
root = config.ResolvePathInConfig(cfg, root)
sfs := module.NewStaticFileServer(name, root, prefix, opts...)
if routerName, ok := cfg["router"].(string); ok && routerName != "" {
    sfs.SetRouterName(routerName)
}

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