Add entry templates for common account types

[coreyleavitt]

Summary

Implements typed template classes for entry creation with full IDE autocompletion and type safety.

Key changes:

• EntryTemplate base class for custom templates (subclass to create your own)
• Built-in templates via Templates namespace: Login, CreditCard, SecureNote, Identity, BankAccount, Server, WirelessRouter, Email, SoftwareLicense, Database
• IconId enum with all 69 KeePass standard icons (PwIcon 0-68)
• Templates carry their field values as typed dataclass fields
• Protected fields via _protected_fields ClassVar
• _include_standard flag controls whether username/password/url are populated

Usage

from kdbxtool import Database, Templates

db = Database.create(password="secret")

# Built-in template with IDE completion
entry = db.root_group.create_entry(
    title="My Visa",
    template=Templates.CreditCard(
        card_number="4111111111111111",
        expiry_date="12/25",
        cvv="123",
    ),
)

# Custom template
from dataclasses import dataclass
from typing import ClassVar
from kdbxtool import EntryTemplate, IconId

@dataclass
class VPNConnection(EntryTemplate):
    _icon_id: ClassVar[int] = IconId.TERMINAL_ENCRYPTED
    _protected_fields: ClassVar[frozenset[str]] = frozenset({"certificate"})

    server: str | None = None
    protocol: str = "OpenVPN"
    certificate: str | None = None

entry = db.root_group.create_entry(
    title="Work VPN",
    template=VPNConnection(server="vpn.company.com"),
)

Test plan

• 26 template tests covering all built-in templates and custom templates
• All 702 existing tests pass
• mypy --strict passes
• ruff check passes

Closes #58