Encode todo add/tag actions as URL-safe base64 JSON payloads

[multiplex55]

Motivation

• Ensure todo:add: and todo:tag: actions can carry arbitrary text and tags (including |, ,, and whitespace) without ambiguous delimiter parsing.
• Make action construction/parsing more robust and future-proof by using a structured payload format.

Description

• Added payload structs TodoAddActionPayload and TodoTagActionPayload and shared encode/decode helpers that serialize to JSON and encode using URL-safe base64. (src/plugins/todo.rs)
• Updated TodoPlugin to serialize add/tag arguments into todo:add:<encoded> and todo:tag:<encoded> when building Action.action. (src/plugins/todo.rs)
• Updated launcher parse_action_kind to decode those payloads and return ActionKind::TodoAdd / ActionKind::TodoSetTags from the deserialized values. (src/launcher.rs)
• Changed ActionKind::TodoAdd.text to own a String so decoded payload text can be carried safely, and adjusted the launch path to call todo::add(&text, priority, &tags). (src/launcher.rs)
• Added unit tests to verify round-trip behavior for texts and tags containing |, ,, and whitespace in both the plugin and launcher parser. (src/plugins/todo.rs, src/launcher.rs)

Testing

• Ran cargo fmt --all which completed successfully.
• Ran cargo test --all-targets but the full test run failed in this environment due to a missing system dependency required by alsa-sys (pkg-config / alsa.pc), so tests could not complete; the added unit tests are included in the tree and will run in CI or on a machine with the ALSA dev package available.

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Codex Task