feat(actions): add SelectNextHalfPage & SelectPrevHalfPage actions

[rudenkornk]

📺 PR Description

Checklist

• my commits and PR title follow the conventional commits format
• if this is a new feature, I have added tests to consolidate the feature and prevent regressions
• if this is a bug fix, I have added a test that reproduces the bug (if applicable)
• I have added a reasonable amount of documentation to the code where appropriate

[Copilot]

Pull request overview

Adds two new navigation actions to move selection by half a page, integrating them into the action enum, UI help relevance filtering, and Television’s action handling/render-trigger logic.

Changes:

• Introduces SelectNextHalfPage / SelectPrevHalfPage in Action with help descriptions.
• Handles the new actions in Television::handle_action by moving the cursor by half the results panel height (channel mode).
• Marks the new actions as relevant for help display and as render-triggering actions.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
television/television.rs	Adds rendering relevance + action handlers for half-page selection movement.
television/screen/help_panel.rs	Includes half-page actions in the “relevant actions” set for help display.
television/action.rs	Adds new Action variants and user-facing descriptions.

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[rudenkornk]

@alexpasmantier may I ask you to take a look on this PR? :)

[rudenkornk]

@alexpasmantier ping :)