mobile + docs: themeColor light-mode, Coach FAB safe-area, Sonner system theme, self-hosting docs

Three hotfix-ready findings from the v1.4.34.3 mobile-deep audit
plus the first two docs A8 audit identified as missing.

Mobile-deep:
  - `viewport.themeColor` was pinned to `#282a36` (dark). The
    Android URL bar + iOS PWA status bar stayed dark even when the
    user was on the light theme. Replaced with the dual-media-query
    form: `#ffffff` under `(prefers-color-scheme: light)`,
    `#282a36` under `(prefers-color-scheme: dark)`. Browser chrome
    now matches the active palette.
  - Coach FAB was pinned at `bottom-20` (5 rem). On notched iPhones
    the home-indicator safe area is ~34 px so the FAB collided with
    the bottom-nav. Moved to
    `bottom-[calc(env(safe-area-inset-bottom,0px)+5rem)]` so it
    floats above the inset-aware nav.
  - Sonner toaster shipped `theme="dark"` hardcoded. On a light-mode
    page the toast contrasted badly against the surface. Switched to
    `theme="system"` so Sonner reads the resolved theme from the
    root element (set pre-paint by the inline theme script in
    layout.tsx) instead of pinning dark.

Self-hosting docs:
  - `docs/self-hosting/getting-started.md` — step-by-step from
    `git clone` to first measurement, covering the 3-secret env
    setup (post-SESSION_SECRET drop), `docker compose up -d`, the
    first-user-becomes-admin contract, and the next-step pointers.
  - `docs/self-hosting/reverse-proxy.md` — Caddy, Traefik, Nginx
    Proxy Manager, Coolify, and bare-Nginx config examples. Closes
    the README link at line 107 that previously punted to the docs
    site.

src/app/layout.tsx

@@ -79,7 +79,14 @@ export const viewport: Viewport = {
   width: "device-width",
   initialScale: 1,
   viewportFit: "cover",
-  themeColor: "#282a36",
+  // Match the browser chrome (Android URL bar, iOS PWA status bar) to
+  // the active palette. The hex values are the resolved background of
+  // `--background` from `app/globals.css` for each theme so the bar
+  // edge never seams against the page on cold paint.
+  themeColor: [
+    { media: "(prefers-color-scheme: light)", color: "#ffffff" },
+    { media: "(prefers-color-scheme: dark)", color: "#282a36" },
+  ],
 };
 
 // Inline script to apply theme before first paint (prevents FOUC)