Add seek command support and refactor controller command API

[kahrendt]

Wires up the player seek commands that were previously recognized but unsendable: SEEK now carries position_ms and SEEK_RELATIVE carries offset_ms, and the controller state parses the server's seek_max_ms.

Refactors send_command onto a flat ClientCommandControllerObject struct built with designated initializers, replacing the positional optional<volume>/optional<mute> parameters (which would have grown a 4th and 5th positional arg for seek). The old positional overload is kept, marked deprecated for removal in v0.8.0, so existing ESPHome/dev-branch callers keep working until the v0.7.0 migration. The vestigial internal ClientCommandControllerObject/ClientCommandMessage structs are removed in favor of the public struct.

Migrates the TUI example to the struct API and add a Shift+Left/Shift+Right relative-seek keybinding.

[Copilot]

Pull request overview

Adds full seek support to the controller role by (1) parsing the server-provided seek_max_ms upper bound and (2) making client seek commands sendable with explicit parameters. It also refactors the controller command-sending API to a single struct-based interface to avoid growing positional optional parameters.

Changes:

• Parse seek_max_ms from server/state controller payloads and expose it via ServerStateControllerObject.
• Refactor format_client_command_message and controller send_command to use ClientCommandControllerObject, adding position_ms/offset_ms serialization for SEEK/SEEK_RELATIVE.
• Update tests, docs, and the TUI example (including Shift+Left/Right relative-seek keybindings) to use the struct API.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/test_protocol.cpp	Adds coverage for seek_max_ms parsing and seek/struct-based command serialization behavior.
src/protocol.cpp	Implements seek_max_ms parsing and struct-based client/command JSON formatting (including seek fields).
src/protocol_messages.h	Removes internal vestigial command message structs; updates client/command formatter declaration to struct-based API.
src/controller_role.cpp	Adds struct-based send_command overload and forwards deprecated positional overload into the struct form.
src/controller_role_impl.h	Updates Impl API to accept ClientCommandControllerObject.
include/sendspin/controller_role.h	Exposes seek_max_ms and defines the public ClientCommandControllerObject (volume/mute/seek params) plus the new send_command overload.
examples/tui_client/tui.cpp	Migrates TUI to struct-based commands and adds Shift+Arrow relative-seek handling.
docs/integration-guide.md	Updates integration documentation to the struct-based command API and documents seek usage + deprecation notice.

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