pre-flight: bandwidth test + persistent config

First-run host launch now offers a one-time upstream measurement
against speed.cloudflare.com/__up via ureq (~5 MB POST, ~5s). The
result lives at ~/.config/pixelpass/config.toml under [bandwidth]
and feeds the default --max-viewers calculation on subsequent runs.

Sticky semantics for the dialog:
- Unmeasured: first-run prompt (Run / Skip)
- Measured / Skipped: silent — never re-prompts
- Failed: ask again on next launch (Retry / give up → Skipped)

`pixelpass --reconfigure` re-runs the test unconditionally for users
whose connection has changed (new ISP, moved house, etc.).

--max-viewers is now Option<u32>. When unset, host startup loads the
saved measurement, runs recommended_max_viewers(safe_mbps, bitrate),
and surfaces the source in the banner: "max viewers : N (auto: X.X
Mbps measured upstream)" — or user-specified / default fallback.

User verified end-to-end on 2026-05-21 16:54 EDT: first-run dialog,
skip path, run path, --reconfigure refresh, and banner integration
all work as expected.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Cargo.lock

@@ -3074,6 +3074,7 @@ dependencies = [
  "anyhow",
  "arboard",
  "ashpd",
+ "chrono",
  "clap",
  "dialoguer",
  "directories",
@@ -3086,8 +3087,10 @@ dependencies = [
  "thiserror 2.0.18",
  "tokio",
  "tokio-util",
+ "toml",
  "tracing",
  "tracing-subscriber",
+ "ureq",
  "uuid",
  "x11rb",
 ]
@@ -4448,6 +4451,34 @@ version = "0.9.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8ecb6da28b8a351d773b68d5825ac39017e680750f980f3a1a85cd8dd28a47c1"
 
+[[package]]
+name = "ureq"
+version = "3.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "dea7109cdcd5864d4eeb1b58a1648dc9bf520360d7af16ec26d0a9354bafcfc0"
+dependencies = [
+ "base64",
+ "log",
+ "percent-encoding",
+ "rustls",
+ "rustls-pki-types",
+ "ureq-proto",
+ "utf8-zero",
+ "webpki-roots",
+]
+
+[[package]]
+name = "ureq-proto"
+version = "0.6.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e994ba84b0bd1b1b0cf92878b7ef898a5c1760108fe7b6010327e274917a808c"
+dependencies = [
+ "base64",
+ "http",
+ "httparse",
+ "log",
+]
+
 [[package]]
 name = "url"
 version = "2.5.8"
@@ -4461,6 +4492,12 @@ dependencies = [
  "serde_derive",
 ]
 
+[[package]]
+name = "utf8-zero"
+version = "0.8.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b8c0a043c9540bae7c578c88f91dda8bd82e59ae27c21baca69c8b191aaf5a6e"
+
 [[package]]
 name = "utf8_iter"
 version = "1.0.4"