Self-gate Set Time/Date on a location fix; drop timezone name from note (follow-up to #508)

[brickbots]

Follow-up to #508. Two changes to the manual Set Time/Date screen.

Approach changed (v2): the location gate now lives inside the screen (self-gating) instead of a menu-layer callback that hard-blocked entry. See ADR 0019.

1. Self-gate entry on a location fix

Manual time is interpreted in the observer's local timezone, which is only derived from a location fix (set_time / set_datetime read location().timezone); without one the entry would localise against a bogus UTC zone (the #508 bug class, see ADR-0018).

Rather than blocking navigation at the menu, the module gates itself: UITimeEntry opens normally, and when there is no location().lock it renders a "Set location first" notice (with a Cancel hint) in place of the entry boxes, keeps its key handlers and its set_time exit-callback inert, and the user backs out with LEFT/Cancel. The check is live, so the boxes appear the instant a fix locks while the screen is open.

UIDateEntry (chained from the time screen, and equally dependent on the zone) carries the same guard. It is only reachable through the gated time screen today, but self-gating keeps it correct if it is ever surfaced directly (e.g. relocated in the menu tree).

Why not hard-block? In text_menu.key_right, a menu item's "callback" is dispatched before its "class" and its return value cancels navigation, whereas a "class" item's pre_callback return value is discarded — so a plain callback is the only way to refuse to open a screen. ADR 0019 records the decision not to: hard-blocking hides the feature and splits the precondition across the menu and the module. Self-gating keeps the feature discoverable and the precondition in one owner. A reusable UIModule.draw_gate_message() renders the notice.

2. Revert the local-time note to a fixed label (unchanged from v1)

#508 built the note as "Enter " + <IANA tz> + " Time", which overruns the 128px panel for long zone names (e.g. America/Argentina/Buenos_Aires). draw_local_time_note now draws _("Enter Local Time") — no timezone name — mirroring UIDateEntry's _("Enter Local Date").

Docs

• ADR 0019 — UI modules self-gate their preconditions (enter-then-back-out), not a menu hard-block.
• docs/ax/ui/CONTEXT.md — defines the Self-gating module term.

Tests

• test_time_date_gate.py (replaces test_callbacks_time_gate.py): drives UITimeEntry/UIDateEntry directly — locked accepts entry and fires the callback; unlocked is inert (digits ignored, no chaining, callback suppressed); the gate message renders.
• test_ui_modules.py: UITimeEntry returns to menu-node discovery (dropped the dynamic-launch entry + spec builder), keeping test_all_ui_modules_covered satisfied. Cold/warm crash-smoke now exercises both the gated and normal render paths.
• test_web_remote_tools.py: refreshed docstrings; added a regression test that the screen opens even with no location seeded (guards against reverting to a hard-block).

Verification

• ruff check + ruff format --check: clean
• mypy PiFinder: no issues (138 files)
• test_ui_modules.py: 222 passed / 2 skipped (hip_main.dat-dependent chart tests)
• pytest -m "smoke or unit": 706 passed
• Visual (headless): confirmed the locked entry screen (boxes + fixed "Enter Local Time" note) and the unlocked gate screen ("Set location / first" + Cancel hint) both render correctly on the 128px panel.

i18n .pot left untouched; the gate reuses the existing Set location\nfirst and  Cancel msgids, so no new strings.

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