Rebuild the interactive configurator on opentui behind a JSON bridge

The TUI is now a TypeScript/opentui app in tui/ rather than Bubble Tea.
opentui is a Zig core with TypeScript bindings and no Go bindings, so this half
of the tool can't live in the Go binary; it compiles with Bun into a sibling
executable (rtsp-streamer-tui) that `rtsp-streamer tui` execs.

Everything that isn't presentation stays in Go, reached over three JSON
commands. The configurator holds no credentials and never writes the config
itself:

  config export           the config, plus limits like max_tiles
  config apply (stdin)    merge cameras/layouts/active_layout, validate, save
                          atomically, reload the daemon
  discover --json         Protect discovery, writing nothing

Two properties of that split are deliberate:

- The controller password never crosses the bridge. It's json:"-" on the way
  out, and apply only merges the three keys the TUI edits, so it can't be
  clobbered on the way back in either.
- apply re-reads the file before merging, so an editor left open for an hour can
  no longer overwrite a `views import`, a `layout set`, or a hand edit made in
  the meantime.

Discovery is previewable as a result: `discover --json` writes nothing, the
merge happens in the TUI, and nothing reaches disk until you save. Only
--enable-rtsp has a side effect, and it's on the controller.

Config structs gain json tags mirroring their yaml ones so the config
round-trips through the bridge under the same key names it has on disk, and
maxGridDim moves to config.MaxGridDim so the CLI and both configurators enforce
one ceiling. The write path is byte-for-byte identical to `layout set`, checked
against a copy of a live config.

Visible change: the grid editor draws real bordered boxes, so a spanning tile is
one box instead of an origin cell plus "·" continuation marks, and the
header-offset arithmetic in mouse.go is gone — the framework hit-tests list
rows. Keybindings, the lipgloss palette and the screen flow are carried over
unchanged; S now saves from anywhere.

The Bubble Tea version stays as `tui --legacy`. It's compiled into the Go binary
and needs no Bun, and on a headless Pi the TUI is the only config UI there is,
so a fallback is worth its weight. The cost of the new one is size: ~120 MB
against ~13 MB, since Bun embeds its runtime and opentui's native library.

Tests: 67 bun tests drive the real (in-memory) opentui renderer, including mouse
click and drag, plus tsc --noEmit. `make test-tui` runs both, and
scripts/preview.ts dumps every screen as text without needing a terminal.

Three bugs found during the port are documented in tui/README.md, since none are
apparent from the code: overlapping cell borders render as ┌ where a lattice
needs ┬; a drag dies after the first resize if the tree is rebuilt, because the
renderer captures the press-target renderable; and a rebuilt box has no computed
layout until the next frame, so its screenX reads 0.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

cmd/rtsp-streamer/tuilaunch.go

@@ -0,0 +1,77 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+)
+
+// tuiBinaryName is the compiled opentui configurator (see tui/ and `make tui`).
+const tuiBinaryName = "rtsp-streamer-tui"
+
+// findTUIBinary locates the opentui configurator. It is a sibling binary rather
+// than something embedded in this one, because opentui is a TypeScript library
+// compiled by Bun.
+//
+// Search order, most specific first:
+//
+//  1. $RTSP_STREAMER_TUI — an explicit path, for development
+//  2. next to this executable — how `make install` lays it out
+//  3. $PATH
+func findTUIBinary() (string, error) {
+	if p := os.Getenv("RTSP_STREAMER_TUI"); p != "" {
+		if isExecutableFile(p) {
+			return p, nil
+		}
+		return "", fmt.Errorf("RTSP_STREAMER_TUI=%s is not an executable file", p)
+	}
+
+	if exe, err := os.Executable(); err == nil {
+		if resolved, err := filepath.EvalSymlinks(exe); err == nil {
+			exe = resolved
+		}
+		if cand := filepath.Join(filepath.Dir(exe), tuiBinaryName); isExecutableFile(cand) {
+			return cand, nil
+		}
+	}
+
+	if p, err := exec.LookPath(tuiBinaryName); err == nil {
+		return p, nil
+	}
+
+	return "", fmt.Errorf(
+		"%s not found — build it with `make tui` (needs Bun) and install it with\n"+
+			"`make install`, or point $RTSP_STREAMER_TUI at it.\n"+
+			"To use the previous Bubble Tea configurator instead, run: rtsp-streamer tui --legacy",
+		tuiBinaryName)
+}
+
+func isExecutableFile(path string) bool {
+	st, err := os.Stat(path)
+	return err == nil && !st.IsDir() && st.Mode()&0o111 != 0
+}
+
+// execTUI runs the configurator with this process's terminal, so it can take
+// raw mode and the alternate screen directly. Its exit status becomes ours.
+func execTUI(ctx context.Context, bin, cfg string) error {
+	c := exec.CommandContext(ctx, bin, "--config", cfg)
+	c.Stdin, c.Stdout, c.Stderr = os.Stdin, os.Stdout, os.Stderr
+	// The TUI shells back to this binary for the bridge commands; tell it which
+	// one, so a locally-built binary is not shadowed by an installed one.
+	env := append(os.Environ(), "RTSP_STREAMER_CONFIG="+cfg)
+	if self, err := os.Executable(); err == nil {
+		env = append(env, "RTSP_STREAMER_BIN="+self)
+	}
+	c.Env = env
+
+	err := c.Run()
+	var exitErr *exec.ExitError
+	if errors.As(err, &exitErr) {
+		// The TUI already reported whatever went wrong on its own terminal.
+		os.Exit(exitErr.ExitCode())
+	}
+	return err
+}