P1-23 / P1-28: base layout, login, session-aware nav + Tailwind build

P1-28: Tailwind standalone CLI wired into the Makefile. `make tailwind`
downloads the pinned v3.4.17 binary into bin/tailwindcss (gitignored),
builds web/styles/input.css → web/static/css/styles.css. `make build`
now runs the CSS pass first; `make tailwind-watch` for dev. Output is
embedded in the binary via web.FS — single static binary, no Node.

The CSS source carries every component class the v1 mockups defined
(status dots, buttons, host row, log viewer, progress bar, fields,
chips, snippet panel, empty state) so screens that land later can
just reach for them.

P1-23: html/template tree at web/templates with two layouts (base
with chrome, chromeless for login + bootstrap), one nav partial, and
two pages (dashboard placeholder, login). internal/server/ui parses
the tree at startup; ui_handlers.go in the http package wires:

  GET  /         dashboard (303 → /login when unauthed)
  GET  /login    sign-in form
  POST /login    consume form, mint session cookie, 303 → /
  POST /logout   drop cookie, 303 → /login
  GET  /static/* embedded Tailwind bundle

The HTML login flow shares store/session logic with /api/auth/login
via a new authenticateAndSession helper — same security guarantees,
two surface representations (HTML form / JSON).

Verified end-to-end: bootstrap → form-login → authed dashboard →
sign-out → 303 cycle works in the browser; Tailwind output emits
only the component classes referenced in the live templates (9.6kB
minified).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/server/http/server.go

@@ -6,7 +6,6 @@ package http
 import (
 	"context"
 	"errors"
-	"fmt"
 	stdhttp "net/http"
 	"time"
 
@@ -15,6 +14,7 @@ import (
 
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/crypto"
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/server/config"
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/server/ui"
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/server/ws"
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/store"
 )
@@ -26,6 +26,10 @@ type Deps struct {
 	Store *store.Store
 	AEAD  *crypto.AEAD
 	Hub   *ws.Hub
+	UI    *ui.Renderer
+	// Version is the binary's build version, surfaced in the chrome.
+	// Empty falls back to "dev".
+	Version string
 	// BootstrapToken (optional, populated only on first run) is the raw
 	// admin-bootstrap token printed in the server logs. While set, the
 	// /bootstrap endpoint accepts it to create the first admin user.
@@ -112,10 +116,18 @@ func (s *Server) routes(r chi.Router) {
 	r.Get("/agent/binary", s.handleAgentBinary)
 	r.Get("/install/*", s.handleInstallAsset)
 
-	// UI handlers will hang off / — Phase 1 will add them.
-	r.Get("/", func(w stdhttp.ResponseWriter, _ *stdhttp.Request) {
-		_, _ = fmt.Fprint(w, "restic-manager — UI not yet implemented")
-	})
+	// Static assets (Tailwind CSS bundle, future favicon).
+	r.Mount("/static/", staticHandler())
+
+	// HTML UI. The renderer is required — fail loud if the binary
+	// was built without templates (impossible in practice given
+	// embed, but guards bad test wiring).
+	if s.deps.UI != nil {
+		r.Get("/", s.handleUIDashboard)
+		r.Get("/login", s.handleUILoginGet)
+		r.Post("/login", s.handleUILoginPost)
+		r.Post("/logout", s.handleUILogoutPost)
+	}
 }
 
 // Start begins listening. Blocks until ListenAndServe returns