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>

tasks.md

@@ -55,12 +55,12 @@ Sizes: **S** = under a day, **M** = 1–3 days, **L** = 3–7 days.
 
 ### UI (HTMX + Tailwind)
 
-- [ ] **P1-23** (M) Base layout, login page, session-aware nav
+- [x] **P1-23** (M) Base layout, login page, session-aware nav
 - [ ] **P1-24** (M) Dashboard: host cards (status dot, last backup, repo size)
 - [ ] **P1-25** (M) Host detail page: snapshots tab + run-now button
 - [ ] **P1-26** (M) Live job log viewer (WS-driven, auto-scroll, cancel button)
 - [ ] **P1-27** (M) "Add host" flow: form takes hostname + repo URL/username/password, mints token (TTL 1h), shows the operator a copy-friendly install command **and** a one-click "download preconfigured installer" — a `install-<hostname>.sh` with `RM_SERVER` + `RM_TOKEN` already templated in (cf. UrBackup Internet-mode push installer). Encrypted repo creds ride on the token row and get pushed to the agent on first WS connect (see secrets/keyring task).
-- [ ] **P1-28** (S) Tailwind build via `tailwindcss` standalone binary (no Node)
+- [x] **P1-28** (S) Tailwind build via `tailwindcss` standalone binary (no Node) — Makefile downloads pinned v3.4.17 into `bin/tailwindcss`, builds `web/styles/input.css` → `web/static/css/styles.css`, embedded into the binary via `web.FS`. `make build` runs Tailwind first.
 
 ### Install scripts