Index Tab selected feedback, no-op guard, page-zero block, and auto-page creation

[No0ne558]

Description:

This PR implements three quality-of-life improvements to Index Tab buttons that together make it dramatically easier to build and navigate multi-page interfaces.

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What changed

Visual feedback — active tab highlights automatically

IndexTabZone::State() is overridden to return the selected state (1) whenever the terminal's current page matches the tab's jump_id. Because Zone::Draw() already uses the State() result to pick frame[state], texture[state], and color[state], the tab for the page you're on renders automatically in its configured selected appearance (edge, texture, font color). No new fields on Terminal are needed.

No-op guard — tapping your current tab does nothing

IndexTabZone::Touch() returns SIGNAL_IGNORED when the terminal is already on the tab's target page. This prevents a pointless re-jump and a screen flash when a user taps the tab for the page they're already viewing.

Block page number zero

ZoneDB::Add(Page*) — the single authoritative insertion point for all pages — now rejects any page with id == 0 with a ReportError. No page numbered zero can be created through any code path.

Auto-create a menu page when a new Index Tab button is saved

ZoneDB::NextAvailablePageID(int start_id, int page_size) is a new helper that walks up from start_id until it finds an unused page ID.

In Terminal::ReadZone(), when a brand-new ZONE_INDEX_TAB is saved with no jump target yet (JUMP_NONE), the system automatically:

1. Finds the first available page ID ≥ 60.
2. Creates a PAGE_ITEM menu page named after the button.
3. Registers it in the zone database.
4. Sets the button's jump to JUMP_NORMAL targeting the new page.

Editing an existing Index Tab that already has a configured jump is completely unaffected.

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Files changed

File	Change
button_zone.hh	Add State() and Touch() overrides to IndexTabZone
button_zone.cc	Implement both overrides
zone.hh	Declare ZoneDB::NextAvailablePageID()
zone.cc	Page-zero guard in ZoneDB::Add(); implement NextAvailablePageID()
terminal.cc	Auto-page-creation block in Terminal::ReadZone()
changelog.md	Entry under [Unreleased] → Added

6 files changed, 73 insertions(+), 1 deletion(-)