feat: implement hashcash proof-of-work for session creation

Add SHA-256-based hashcash proof-of-work requirement to POST /session
to prevent abuse via rapid session creation. The server advertises the
required difficulty via GET /server (hashcash_bits field), and clients
must include a valid stamp in the X-Hashcash request header.

Server-side:
- New internal/hashcash package with stamp validation (format, bits,
  date, resource, replay prevention via in-memory spent set)
- Config: NEOIRC_HASHCASH_BITS env var (default 20, set 0 to disable)
- GET /server includes hashcash_bits when > 0
- POST /session validates X-Hashcash header when enabled
- Returns HTTP 402 for missing/invalid stamps

Client-side:
- SPA: fetches hashcash_bits from /server, computes stamp using Web
  Crypto API with batched SHA-256, shows 'Computing proof-of-work...'
  feedback during computation
- CLI: api package gains MintHashcash() function, CreateSession()
  auto-fetches server info and computes stamp when required

Stamp format: 1:bits:YYMMDD:resource::counter (standard hashcash)

closes #11

internal/config/config.go

@@ -44,6 +44,7 @@ type Config struct {
 	ServerName         string
 	FederationKey      string
 	SessionIdleTimeout string
+	HashcashBits       int
 	params             *Params
 	log                *slog.Logger
 }
@@ -74,6 +75,7 @@ func New(
 	viper.SetDefault("SERVER_NAME", "")
 	viper.SetDefault("FEDERATION_KEY", "")
 	viper.SetDefault("SESSION_IDLE_TIMEOUT", "24h")
+	viper.SetDefault("NEOIRC_HASHCASH_BITS", "20")
 
 	err := viper.ReadInConfig()
 	if err != nil {
@@ -98,6 +100,7 @@ func New(
 		ServerName:         viper.GetString("SERVER_NAME"),
 		FederationKey:      viper.GetString("FEDERATION_KEY"),
 		SessionIdleTimeout: viper.GetString("SESSION_IDLE_TIMEOUT"),
+		HashcashBits:       viper.GetInt("NEOIRC_HASHCASH_BITS"),
 		log:                log,
 		params:             &params,
 	}