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>

Makefile

@@ -10,17 +10,41 @@ GOFLAGS        := -trimpath
 DOCKER_IMAGE   ?= ghcr.io/dcglab/restic-manager
 DOCKER_TAG     ?= dev
 
-.PHONY: help build server agent test test-race lint fmt tidy clean run-server run-agent docker release
+# Tailwind standalone CLI — single binary, no Node toolchain.
+# See spec.md §4.1 / tasks.md P1-28 for why.
+TAILWIND_VERSION  ?= v3.4.17
+TAILWIND_OS       := $(shell uname -s | tr A-Z a-z)
+TAILWIND_ARCH     := $(shell uname -m | sed 's/x86_64/x64/;s/aarch64/arm64/')
+TAILWIND_BIN      := $(BIN_DIR)/tailwindcss
+TAILWIND_URL      := https://github.com/tailwindlabs/tailwindcss/releases/download/$(TAILWIND_VERSION)/tailwindcss-$(TAILWIND_OS)-$(TAILWIND_ARCH)
+TAILWIND_INPUT    := web/styles/input.css
+TAILWIND_OUTPUT   := web/static/css/styles.css
+
+.PHONY: help build server agent test test-race lint fmt tidy clean run-server run-agent docker release tailwind tailwind-watch
 
 help:
 	@grep -E '^[a-zA-Z_-]+:.*?## ' $(MAKEFILE_LIST) | awk 'BEGIN{FS=":.*?## "};{printf "  \033[36m%-14s\033[0m %s\n",$$1,$$2}'
 
-build: server agent ## Build server + agent into ./bin
+build: tailwind server agent ## Build server + agent into ./bin (incl. Tailwind CSS)
 
 server: ## Build the server binary
 	@mkdir -p $(BIN_DIR)
 	CGO_ENABLED=0 go build $(GOFLAGS) -ldflags "$(LDFLAGS)" -o $(SERVER_BIN) ./cmd/server
 
+$(TAILWIND_BIN):
+	@mkdir -p $(BIN_DIR)
+	@echo "==> downloading tailwindcss $(TAILWIND_VERSION) ($(TAILWIND_OS)/$(TAILWIND_ARCH))"
+	curl -fsSL -o $@ "$(TAILWIND_URL)"
+	chmod +x $@
+
+tailwind: $(TAILWIND_BIN) ## Build the CSS bundle from web/styles/input.css
+	@mkdir -p $$(dirname $(TAILWIND_OUTPUT))
+	$(TAILWIND_BIN) -c tailwind.config.js -i $(TAILWIND_INPUT) -o $(TAILWIND_OUTPUT) --minify
+
+tailwind-watch: $(TAILWIND_BIN) ## Watch and rebuild on every save
+	@mkdir -p $$(dirname $(TAILWIND_OUTPUT))
+	$(TAILWIND_BIN) -c tailwind.config.js -i $(TAILWIND_INPUT) -o $(TAILWIND_OUTPUT) --watch
+
 agent: ## Build the agent binary
 	@mkdir -p $(BIN_DIR)
 	CGO_ENABLED=0 go build $(GOFLAGS) -ldflags "$(LDFLAGS)" -o $(AGENT_BIN) ./cmd/agent
@@ -42,7 +66,7 @@ tidy: ## go mod tidy
 	go mod tidy
 
 clean: ## Remove build artifacts
-	rm -rf $(BIN_DIR) coverage.out coverage.html
+	rm -rf $(BIN_DIR) coverage.out coverage.html $(TAILWIND_OUTPUT)
 
 run-server: server ## Build and run the server
 	$(SERVER_BIN)