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>

src/cli.rs

@@ -46,9 +46,11 @@ pub struct Cli {
     pub low_latency: bool,
 
     /// Maximum number of concurrent viewers. Additional connections are
-    /// politely refused with a "host full" message.
-    #[arg(long, default_value_t = 2)]
-    pub max_viewers: u32,
+    /// politely refused with a "host full" message. Defaults to the
+    /// connection-aware recommendation from the bandwidth pre-flight if
+    /// available, otherwise 2.
+    #[arg(long)]
+    pub max_viewers: Option<u32>,
 
     // ── viewer options ────────────────────────────────────────────────
     /// Local TCP port for the viewer to expose (default: random).
@@ -63,6 +65,12 @@ pub struct Cli {
     /// Clean up orphaned PipeWire state from a crashed host run, then exit.
     #[arg(long)]
     pub repair: bool,
+
+    /// Re-run the bandwidth pre-flight test, save the result, then exit.
+    /// Use this if your connection has changed (new ISP, moved house, etc.)
+    /// or if the previously saved test result is stale.
+    #[arg(long)]
+    pub reconfigure: bool,
 }
 
 #[derive(ValueEnum, Clone, Copy, Debug)]
@@ -81,7 +89,7 @@ pub struct HostOpts {
     pub framerate: u32,
     pub no_hwencode: bool,
     pub low_latency: bool,
-    pub max_viewers: u32,
+    pub max_viewers: Option<u32>,
     pub interactive: bool,
 }