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.toml

@@ -30,6 +30,9 @@ uuid = { version = "1", features = ["v4"] }
 iroh-tickets = "1.0.0-rc.0"
 dialoguer = { version = "0.12", default-features = false }
 arboard = { version = "3", default-features = false, features = ["wayland-data-control"] }
+ureq = { version = "3", default-features = false, features = ["rustls"] }
+toml = "1"
+chrono = { version = "0.4", default-features = false, features = ["clock", "serde"] }
 
 [profile.release]
 lto = "thin"