[Feature] Add a lightweight brand logo, favicon, and social image

[maquchizi]

Problem

ProbeView has no visual identity. The header is text-only ({siteName} +
"Status"), the favicon is the stock Next.js default (app/favicon.ico), and
there is no social/OG image, apple-touch-icon, or theme-color, so shared links
preview as bare text and the browser tab/mobile chrome are unbranded. The repo
also still carries the default Next.js placeholder SVGs (public/next.svg,
vercel.svg, file.svg, globe.svg, window.svg).

Proposed solution

Introduce a small, hardcoded SVG product mark for ProbeView (not the
deployment's siteName) so it stays white-label / self-host-safe, reused
everywhere it makes sense:

• A dependency-free SVG mark component matching the existing
  components/icons.tsx family (viewBox="0 0 24 24", currentColor,
  strokeWidth="2", round caps/joins) that adapts to the light/dark theme.
• Place the mark in the header lockup (app/layout.tsx) beside siteName.
• Favicon via App Router app/icon.svg, replacing the stock app/favicon.ico.
• Social image via app/opengraph-image and an app/apple-icon.
• theme-color (light/dark) through the viewport export.
• Remove the unused public/*.svg placeholders.

The exact glyph is left to the implementer, provided it reads clearly at 16px,
works in both themes via currentColor, and keeps the compact status-page feel.

Alternatives considered

• A siteName-specific logo — rejected because it breaks the white-label /
  self-host design; branding should stay portable and env-driven.
• Committing binary .ico/.png assets — a static SVG (plus generated
  opengraph-image/apple-icon) is lighter and theme-aware.
• Making the mark configurable now — deferred; hardcoded is enough for now.

Additional context

• Keep credentials server-only and behavior read-only; this is presentation
  only, no new runtime state.
• Palette lives in app/globals.css (status green #22c55e); the mark should
  lean on currentColor rather than coupling to it.
• Update docs/ if any user-visible branding/config expectations change.

Note: this project is intentionally a read-only live uptime/latency view
with no incidents feature. Proposals that keep it stateless and configurable
by environment are most likely to be accepted.