Add Cashu ecash support

[kravens]

What changed

Adds optional Cashu ecash support to BitTipBot. Users can mint, send, and redeem bearer ecash tokens backed by an external Cashu mint, alongside the existing LNbits Lightning wallet.

New

• internal/cashu/ — a minimal mint client (client.go), blinded-message crypto (crypto.go), token serialization (token.go), and shared types (types.go).
• /cashu command (internal/telegram/cashu.go) with subcommands:
  • mint <amount> [memo] — pay the mint's Lightning invoice to create ecash
  • send <amount> — produce a Cashu token to hand out
  • receive / redeem / claim <token> — swap a token back into the wallet
• Inline sharing (internal/telegram/cashu_inline.go) — share a claimable Cashu token in a group chat via inline query, with Claim / Cancel buttons.

Wiring

• cashu config block (enabled, mint_url, unit) with validation in internal/config.go; example in config.yaml.example.
• CashuClient added to TipBot, constructed only when enabled.
• Handler + inline callback registration in handler.go, inline-query entry in inline_query.go.
• Cashu-specific error types in internal/errors/types.go and English strings in translations/en.toml.

Why

Lets the bot custody and transfer value as Chaumian ecash (better privacy, instant offline-style transfers) without replacing the existing Lightning/LNbits flow.

Notes for reviewers

• Disabled by default. With cashu.enabled: false (the default), no client is constructed and every code path is a no-op — zero impact on existing deployments.
• Mint URL is normalized (trailing slash trimmed) and unit defaults to sat.
• go build ./... passes.
• Only English translations are added; other locales fall back to defaults.
• The mint client implements the subset of NUTs needed for mint/melt/swap; it has not yet been exercised against every mint implementation — mint.minibits.cash is the example default.

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