refactor: introduce slog for operational logging, keep ui.UI for presentation

Context

The codebase currently has two output channels:

• ui.UI (356 calls) — CLI presentation layer with colors, spinners, prompts, JSON mode, TTY detection (34 methods)
• log.* (99 calls) — server binaries (x402-verifier, x402-buyer) using stdlib log

These work well for their respective contexts. The gap appears when internal helpers (e.g., fixVolumeOwnership) need to report operational warnings — they currently require threading *ui.UI through the entire call chain even though they have no presentation concerns.

Problem

There's a semantic mismatch between two types of output:

Type	Example	Right channel
Presentation	"Wallet generated ✓", spinner, prompt	ui.UI
Operational	"chown failed for /data/x: permission denied"	Should be slog

Today both go through ui.UI, which means:

• Presentation layer leaks into deep internals
• Every function in the chain needs *ui.UI even if it only warns on failure
• Server binaries can't share internal packages that use ui.UI

Proposal

Introduce log/slog (stdlib since Go 1.21, project requires 1.25+) as the operational logging backbone:

CLI (cmd/obol/main.go)

// Pretty handler for human-friendly terminal output
slog.SetDefault(slog.New(newPrettyHandler(os.Stderr, verbose)))

Server binaries (cmd/x402-verifier/, cmd/x402-buyer/)

// Structured JSON handler for container logs
slog.SetDefault(slog.New(slog.NewJSONHandler(os.Stderr, nil)))

Internal helpers

// No *ui.UI threading needed — just call slog
func fixVolumeOwnership(cfg *config.Config, hostPath string) {
    if err := cmd.Run(); err != nil {
        slog.Warn("chown failed", "path", hostPath, "err", err)
    }
}

Keep ui.UI for

• u.Success(), u.Info() — user-facing status messages
• u.RunWithSpinner() — progress indicators
• u.Confirm(), u.Select() — interactive prompts
• u.JSON() — structured output mode

When to do this

This becomes important when:

• The ServiceOffer controller (#296) lands and shares internal packages with CLI code
• Server binaries need to share code from internal/ that currently requires *ui.UI
• Observability requirements grow (structured log aggregation, field-based filtering)

Not urgent today — the ui.UI threading approach works. This is a future improvement.

Scope

• Add internal/logging/ with pretty handler (CLI) and JSON handler (servers)
• Migrate log.Printf in server binaries → slog.*
• Migrate operational warnings in internals (e.g., fixVolumeOwnership) → slog.Warn
• Keep all 356+ u.* calls for presentation output
• Document "when to use slog vs ui.UI" in CLAUDE.md

Start with cmd/obol/main.go, cmd/x402-verifier/, cmd/x402-buyer/, and the proposed internal/logging/ package. Review existing log.Printf calls, operational warnings such as fixVolumeOwnership, and the ui.UI usage before defining handler setup. Done means server logs use slog, internal operational warnings no longer require *ui.UI, presentation calls remain unchanged, and the slog-versus-ui.UI guidance is documented in CLAUDE.md.